Understanding Development by Minelli Alessandro

Author:Minelli, Alessandro
Language: eng
Format: epub
Publisher: Cambridge University Press
Published: 2021-03-26T00:00:00+00:00

Closing the Circle

The Egg Is Not the Least Specialized of Cells

Three weeks after the start of incubation, the chick is ready to break through the eggshell and to begin active life. By interrupting incubation at different times, we could have seen, in the previous days, the progressive emergence of the embryo and its subsequent development into chick. At the time the egg was laid, however, the egg contained only dense fluids, with no evidence of the future animal.

For centuries since Aristotle’s pioneering studies, the chicken’s egg has been the most accessible object on which to study an animal’s early development. It is understandable, therefore, that the very notion of development has long been based on observations made on this bird. To the eye of the observer, the chick takes form gradually. At the beginning, no sign seems to pre-exist of the complex structure that will emerge gradually. Moreover, the isolation of the embryo inside the shell excludes any shaping intervention from outside. Chick development therefore supports the theory of development by epigenesis. To understand the meaning of this expression (literally, genesis by additions) it should be said that the term ‘epigenesis’ is used here in the meaning that has been attributed to it for over 2000 years; it should not be confused with the epigenetics of recent decades (p. 54). The epigenetic theory of development is the alternative to pre-existence (p. 81).

Regardless of the conflicting interpretations of the nature of developmental phenomena, advances have reinforced the notion that the starting point for the development of a new individual is the egg. For a long time, uncertainty remained about mammals, which are viviparous and thus did not provide easy access to the earliest stages of their individual development. Do they also produce eggs? The answer to this question was provided in 1827 by Karl Ernst von Baer, who described the ovum of the dog. The egg of the human species would be described a century later. The fertilization of an egg by a spermatozoon was observed for the first time in 1875, by the German zoologist Oskar Hertwig.

Shortly after von Baer’s discovery, the cellular theory of Schleiden and Schwann took hold (p. 43). At this point, it seemed natural to generalize about two points: first, the development of all animals starts from a particular cell, the egg; second, the egg is a sort of universal (generalized and perhaps ancestral) cell, devoid of all the specializations that characterize the different types of cells that make up an animal’s body. A little later, with the emergence of the theory of evolution, the egg, the single cell from which development begins, would start to be compared to the single-celled ancestors of all multicellular animals.

These two generalizations, however, are not justified. The first neglects those animals that are formed from buds or other parts of the parent’s body (p. 76); the second attributes to the egg prerogatives that do not belong to it. I address this rather serious problem in the following paragraphs.

First of all, eggs are not all the same.

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