Embryology

EMBRYOLOGY. - Considered in the strict sense, embryology should deal with the development of the organism until it is contained within the membranes of the egg and is not capable of leading a free life there, that is, until it draws its energy requirements from the material of the egg itself. But in practice embryology also deals with the subsequent phase up to the attainment of the definitive condition of the adult. In this phase the organism may pass through various conditions of existence such as are represented by the larval stages (even several successive ones living in different environments) or by those of the fetus of viviparous animals, in which the offspring enters into relations with the mother, and by the juvenile stages in which, although resembling the adult, the processes of growth and organic development continue.

Embryology can be the expression of the study of the development of a single organism as well as of the comparative study of the development of different species (comparative embryology); it can concern itself only with structure (descriptive embryology) as well as with the functionality of the different organs during their ontogenesis (and physiological embryology). The comparative point of view of embryology received a very great impulse in the first half of the 16th century when, under the influence of Darwin and Von Baer, an attempt was made to find in the history of development both a documentation and the key to evolutionary history; Haeckel's aphorism "ontogeny recapitulates phylogeny" indicates in the most concise way the spirit of this research. However, even if the hopes in this direction were disappointed, this approach was precious for the great harvest of observations that sprang from it. Today, modern comparative embryology does not see in the succession of embryonic stages a memory of past conditions, nor does it claim to explain the origin of man, but it sees, through the comparison of the stages of development, how upon a common structural model « great groupings of living beings » development reaches different differentiations in relation to the future functional needs connected with the particular conditions of life in different environments (ecological embryology). The presence of the branchial pouches in the human embryo, by way of example, is not a demonstration that man has passed in his evolution through a fish-like branchiate condition, but objectively tells us that man and fishes, and in a word all the animals that present branchial pouches in their development, are built upon a common structural, morphological plan. Only that, while in fishes the branchial pouches are transformed into clefts with gills in relation to their particular life in water, in man the pouches give origin to glands of internal secretion and to accessory apparatuses of the auditory organ such as are necessary to the particular condition of life of this organism. Comparative embryology thus gives the morphological meaning of many structures and explains the value of many so-called rudimentary organs and places on a scientific plane the study of malformations and anomalies known as arrests of development and clarifies the nature of many teratological conditions.

But modern embryology is not satisfied with these data, even though comparison remains the fundamental method of biological research. By means of experiment it seeks to understand the causes that lead to a determined sequence of stages or lead an embryonic rudiment to follow a determined path of development: this is the causal approach of experimental embryology. This approach was initiated in 1893, with the founding of the famous journal on the mechanics of development (Archiv für Entwicklungsmechanik der Organismen) by Wilhelm Roux, and continued up to the present time by very many scholars among whom the names of Driesch, Spemann, Harrison stood out.

Particularly in embryology studied by this method the two opposite tendencies of the spirit of scholars have manifested themselves: the mechanistic and the vitalistic. Thus, while the mechanist sees in development a sequence of reactions determined by causal factors of a chemical and physical nature with an eminently epigenetic process, the vitalist sees in development the externalization of a total potentiality already contained in the egg and which is put into action according to a preceding path and with an established end. However, the modern vitalist does not conceive preformation as it was held by the ancient ovists or animalculists of the 18th century, who believed the individual already completely formed in the egg or in the sperm, but very small and susceptible only of a growth comparable to a swelling, but he holds as pre-existing the power of this future individual, that is, his entelechy, in the sense given by Driesch to this Aristotelian word, that is, of that potentiality that makes two eggs, apparently similar, as different as the respective adults originating from them are different. The apparent irreconcilable dissension of these two tendencies is today overcome as regards the « way » in which development takes place, and even the vitalist who accepts the concept of an intrinsic vital principle, animating matter, is convinced that such a power is externalized through an epigenetic development, whereby every stage of development is relative and influenced by factors that have come to be individuated in the preceding stage. Therefore experimental embryology gives many examples of the possibility of modifying the course of development if these factors are in any way altered. Many anomalies of development, which range from monstrosities due to hyperformation, to duplication, etc., to Siamese twins, to the physiologically normal condition of monovular twins, are perfectly explained by their experimental reproduction (see also ANIMATION).

(courtesy of Prof. A. Stefanetti)
EMBRYOLOGY - Human embryos (from the plates of His). The numbers indicate the age in days.

The fact that from a single egg two or more embryos, and therefore complete individuals, could be obtained seemed the best documentation against the preformationist idea and the idea of potency, which is its modern exponent. But these experiments do not at all destroy such concepts, even if modern authors wish too easily to translate phenomena of life into pure chemical and physical expressions. The more or less profound alterations that experiment provokes serve instead to demonstrate (as, for example, the production of several individuals from a single egg) the manifestation of this potency which tends, even under altered experimental conditions (or accidental ones, in cases that also occur in nature), reacting to such abnormal factors, to bring forth, in the manner that matter permits, that "whole" which is in potency in the egg (holistic conception of development). The egg, after it has been constricted by the experimenter's silk thread, demonstrates how its two halves, rendered independent, independently bring forth, regulating themselves, that unitary whole which was previously in the undivided egg. But if the separation is incomplete, and there therefore remains a physical obstacle to the free action of the two parts, or if other altering factors come into play, or in any case a reciprocal action remains, the regulatory activity, which nevertheless manifests itself, fails to achieve its purpose and a monstrosity is formed, in the vain attempt hindered by the too altered conditions of the ovular or embryonic material.

It has been brought to light by Spemann, through experiments on amphibian embryos, how not all parts of the egg have the same value, but how centers with preponderant or directive activity (organizer centers) become established: to a primary organizer center that determines the organization of the axial organs of the embryo (dorsal cord, neural tube, etc.) there succeed more numerous secondary organizers, which induce organogenesis in more limited morphogenetic fields, and then tertiary, quaternary organizers, etc., progressively in more peripheral territories and with more specialized action. It has been possible to demonstrate how the fertilized egg is not homogeneous and how often, besides having a distribution of substances along the polar axis, it comes to assume, through the unequal distribution of substances, also a bilateral symmetry; such anisotropy of the egg is more or less intense and more or less precocious in the various animal species. With the segmentation of the egg into successive generations of cells (blastomeres) there comes about a segregation of these substances in the various blastomeres, which come to assume a different value from a chemical and physicochemical point of view. With such segregation there is a gradual loss of the possibility of bringing forth the total potency of the various blastomeres, assuming they are isolated, through the lack of all the chemical components that are necessary. Thus even the cells of the most differentiated tissues of the adult, although they preserve from a theoretical point of view the total idea of the organism (modern views on the nucleus, depositary of hereditary characters, fit within this concept) are far from the possibility of realizing it through the lack of all those material ingredients present in the egg and lost with segregation, and through the profound modification of the cytoplasm in which specific differentiation takes place (v. DIFFERENZIA NEXTO BIOLOGICO). The agamic, somatic reproduction of many lower animal and plant organisms with relative differentiation stands as proof of this concept. The chemistry of cells is therefore of great importance. The experiments of Holtfrether, of Needham and many others have brought to light how even the action of organizers, localized as has been said in specific cell groups, is due to the particular chemistry of these elements. Attempts have also been made to specify the chemical substance responsible for organization (determination), and sterols, nucleic acids, special proteins, fatty acids, etc., have in turn demonstrated their importance. But all these substances, if they demonstrate an action, never demonstrate it as the living organizer does. It is not the chemical substance that is the basic factor of determination, but it, and they, being certainly multiple, are nothing but the material means by which the living cell of the center exercises its action, proper to that study of devel-

(courtesy of Prof. A. Stefanelli)
EMBRYOLOGY - Anisotropy of the egg, segregation, primary organizer and fields of secondary organization in amphibians.

development and according to a precise purpose. The antagonism between those biologists who want to conceive life and its becoming in development as a sequence of chemical and physico-chemical reactions, governed by the same laws of the physical world, and those who limit the application of such laws to the merely material part that constitutes living beings, recognizing in life and in development other [forces] that transcend it, is always open, but of purely speculative value, since, from the practical side, both work with the same means and on the same matter on which life throbs, because only its matter is subject to scientific experience.

The mechanists say that the vitalist idea removes every incentive to research since, for example, in the embryological field, development would be nothing but the actualization of a design, of a potency, of an entelechy of immaterial character and as such unreachable by experience. But in truth, if such a conception of life places its first cause beyond experience, it does not at all limit research (the only one, in the end, that can be done) into how phenomena unfold and into the factors that come into play, by epigenetic mechanism, in the externalization of the formative potency.

Alberto Stefanelli