EMBRIOLOGIA

EMBRYOLOGY. — Strictly speaking, embryology should deal with the development of the organism as long as it is contained within the membranes of the egg and is incapable of leading an independent life, that is, until it draws its energy supply from the material of the egg itself. In practice, however, embryology also concerns itself with the subsequent phase up to the attainment of the definitive condition of the adult. During this phase, the organism may pass through various conditions of existence, such as those represented by larval stages (which may be successive and may live in different environments) or those of the fetus in viviparous animals, in which the offspring establishes a relationship with the mother, and by juvenile stages in which, although resembling the adult, the processes of growth and organic development continue.

Embryology may be the study of the development of a single organism or a comparative study of the development of different species (comparative embryology); it may focus solely on structure (descriptive embryology) or on the functionality of the various organs during their ontogenesis (physiological embryology). The comparative viewpoint of embryology received a great impetus in the first half of the 19th century, when, under the influence of Darwin and von Baer, researchers sought in the history of development a documentation and a key to evolutionary history. Haeckel’s biogenetic law—“ontogeny recapitulates phylogeny”—most succinctly expresses the spirit of this research. However, even though hopes in this direction were disappointed, this approach proved valuable for the vast body of observations it generated. Today, modern comparative embryology does not view the sequence of embryonic stages as a record of past conditions, nor does it claim to explain the origin of man. Instead, by comparing developmental stages, it reveals how, on the basis of a common structural model, large groupings of living beings achieve different developments in relation to future functional needs linked to the particular conditions of life in different environments (ecological embryology). The presence of branchial pouches in the human embryo, for example, does not demonstrate that man passed through a gill-bearing phase in his evolutionary history; rather, it objectively shows that man and fish—and, in short, all animals that develop branchial pouches—are constructed according to a common structural and morphological plan. While in fish the branchial pouches develop into gill slits in relation to their aquatic life, in man they give rise to glands, internal secretions, and accessory structures of the auditory organ, which are necessary for the particular conditions of life of this organism. Comparative embryology thus elucidates the morphological significance of many structures, explains the value of many so-called rudimentary organs, places the study of malformations and anomalies known as developmental arrests on a scientific footing, 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. Through experimentation, it seeks to understand the causes that lead to a given sequence of stages or cause an embryonic Anlage to follow a particular developmental pathway: this is the causal approach of experimental embryology. This line of research was initiated in 1893 with the founding of the famous journal on developmental mechanics, and even vitalists, who accept the concept of an intrinsic vital principle animating matter, are convinced that such a principle manifests itself through epigenetic development, whereby each stage of development is relative and influenced by factors that have emerged in the preceding stage. Experimental embryology thus provides many examples of the possibility of altering the course of development if these factors are in any way disrupted. Many developmental anomalies, ranging from monstrosities due to hyperformation or duplication to Siamese twins and the physiologically normal condition of monovular twins, are perfectly explained by their experimental reproduction (see also ANIMATION).

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**Fig. 1**

The fact that one egg can give rise to two or more embryos, and thus complete individuals, seemed the best evidence against preformationist ideas, the modern exponent of which is the concept of potency. But these experiments do not in any way destroy such concepts, even if modern authors too readily translate phenomena of life into mere chemical and physical expressions. The more or less profound alterations induced by experimentation instead demonstrate (as, for example, the production of multiple individuals from a single egg) the manifestation of this potency, which, even under altered experimental (and accidental, in cases that also occur in nature) conditions, reacts to such abnormal factors by expressing, to the extent that the matter allows, that “whole” which is potentially present in the egg (holistic conception of development). When the egg is constricted by the silk thread of the experimenter, it demonstrates how its two halves, rendered independent, independently and regulatively express that unitary whole which previously existed in the undivided egg. But if the separation is incomplete and a physical obstacle remains to the free action of the two parts, or if other altering factors come into play, or if mutual interaction persists, the regulative activity, though it may manifest itself, fails to achieve its goal, and a monstrosity results in the vain attempt hindered by the excessively altered conditions of the ovular or embryonic material.

Spemann demonstrated through experiments on amphibian embryos that not all parts of the egg have the same value, but that centres with predominant or directive activity (organizer centres) are established: a primary organizer centre determines the organization of the axial organs of the embryo (dorsal cord, neural tube, etc.), followed by more numerous secondary organizers, which induce organogenesis in more limited morphogenetic fields, and then tertiary, quaternary, etc., organizers, progressively in more peripheral territories with increasingly specialized action. It has been shown that the fertilized egg is not homogeneous and that, in addition to a distribution of substances along the polar axis, it often acquires, through the unequal distribution of substances, a bilateral symmetry; this anisotropy of the egg is more or less intense and occurs earlier or later in different animal species. With the segmentation of the egg into successive generations of cells (blastomeres), these substances become segregated into the various blastomeres, which thus acquire different chemical and physico-chemical properties. Through this segregation, the blastomeres gradually lose the potential to express their total potency if isolated, due to the lack of all the chemical components necessary. Thus, even the cells of the most differentiated tissues of the adult, although they theoretically retain the total idea of the organism (modern views on the nucleus as the repository of hereditary characters fit into this concept), are far from being able to realize it due to the lack of all those material ingredients present in the egg and lost through segregation, and due to the profound modification of the cytoplasm in which specific differentiation occurs (cf. BIOLOGICAL DIFFERENTIATION). The agamic, somatic reproduction of many lower animals and plants, with their related differentiation, serves as evidence for this concept. The chemistry of the cells is therefore of great importance. The experiments of Holtfreter, Needham and many others have shown that even the action of organizers, localized as it is in specific cell groups, is due to the particular chemistry of these elements. Attempts have also been made to identify the chemical substance responsible for organization (determination): sterols, nucleic acids, special proteins, fatty acids, etc., have successively demonstrated their importance. But all these substances, while showing an action, never do so like the living organizer. It is not the chemical substance that is the basic factor of determination, but rather, being certainly multiple, they are merely the material means by which the living cell of the centre exerts its action, which is proper to that stage of development.