PART I
THE INDIVIDUAL ORGANISM WITH REGARD TO
FORM AND METABOLISM
A. ELEMENTARY MORPHOGENESIS
Evolutio and Epigenesis in the old Sense
The organism is a specific body, built up by a typical combination of specific and different parts. It is implied in the words of this definition, that the organism is different, not only from crystals, as was mentioned in the last lecture, but also from all combinations of crystals, such as those called dendrites and others, which consist of a typical arrangement of identical units, the nature of their combination depending on the forces of every single one of their parts. For this reason dendrites, in spite of the typical features in their combination, must be called aggregates; but the organism is not an aggregate even from the most superficial point of view.
We have said before, what must have been familiar to you already, that the organism is not always the same in its individual life, that it has its development, leading from simpler to more complicated forms of combination of parts; there is a “production of visible manifoldness” carried out during development, to describe the chief character of that process in the words of Wilhelm Roux. We leave it an open question in our present merely descriptive analysis, whether there was already a “manifoldness,” in an invisible state, before development, or whether the phrase “production of manifoldness” is to be understood in an absolute sense.
It has not always been granted in the history of biology, and of embryology especially, that production of visible manifoldness is the chief feature of what is called an organism’s embryology or ontogeny: the eighteenth century is full of determined scientific battles over the question. One school, with Albert von Haller and Bonnet as its leading men, maintained the view that there was no production of different parts at all in development, this process being a mere “evolutio,” that is, a growth of parts already existing from the beginning, yes, from the very beginning of life; whilst the other school, with C. F. Wolff and Blumenbach at its head, supported the opposite doctrine of so-called “epigenesis,” which has been proved to be the right one.
To some extent these differences of opinion were only the outcome of the rather imperfect state of the optical instruments of that period. But there were also deeper reasons beyond mere difficulties of description; there were theoretical convictions underlying them. It is impossible, said the one party, that there is any real production of new parts; there must be such a production, said the other.
We ourselves shall have to deal with these questions of the theory of organic development; but at present our object is narrower, and merely descriptive. It certainly is of great importance to understand most clearly that there actually is a “production of visible manifoldness” during ontogenesis in the descriptive sense; the knowledge of the fact of this process must be the very foundation of all studies on the theory of development in any case, and therefore we shall devote this whole lecture to studies in merely descriptive embryology.
But descriptive embryology, even if it is to serve merely as an instance of the universality of the fact of epigenesis, can only be studied successfully with reference to a concrete case. We select the development of the common sea-urchin (Echinus microtuberculatus) as such a case, and we are the more entitled to select this organism rather than another, because most of the analytical experimental work, carried out in the interests of a real theory of development, has been done on the germs of this animal. Therefore, to know at least the outlines of the individual embryology of the Echinus may indeed be called the conditio sine qua non for a real understanding of what is to follow.