82 PRIMITIVE FORMS OF LIFE 



and nothing else. Each part corresponds to a particular portion 

 of the egg, and even the substance of the egg itself is sub- 

 divided in such a way that we can say it possesses an anterior 

 and posterior end, a dorsal and ventral region, and a left and 

 right side, corresponding exactly to the equivalent parts of 

 the embryo. But, between this kind of egg and an egg capable 

 of producing numerous embryos, there are all kinds of inter- 

 mediate forms. This determination of the functions of the 

 blastomeres, by virtue of which, as soon as each becomes an 

 individual unit, it is destined to occupy a place in a definite 

 portion of the body, which it cannot then leave and for which 

 it must form tissues and organs, does not, of course, represent 

 the initial condition of all embryogenetic development. It 

 has been gradually evolved step by step, giving to embryo- 

 genetic phenomena a precision which leads in turn to a 

 maximum rapidity in development. This is merely one 

 particular aspect of the phenomena of embryogenetic acceleration 

 or tachy 'genesis, which have played so great a part in the 

 evolution of organisms, and which have been so long neglected, 

 though it is impossible to understand anything about the 

 history of living forms or the determination of their origin 

 without taking these phenomena into consideration. It is 

 quite evident that if, reversing the actual order of the facts, 

 we consider the normal embryological type to be an egg in which 

 everything is determined beforehand, nothing but a miracle 

 could have created it, and we should not need to look further for 

 explanation. This notion has frequently recurred with fatal 

 results in the philosophic biology which either takes man as its 

 point of departure or as the model for the entire animal 

 kingdom. But just as there are some blastomeres in the egg 

 which are destined for different functions according to the rank 

 they occupy in the sequence of the egg's bipartitions and to the 

 position this imposes, so there are others which are not of 

 direct service in building up the body and are held in reserve, 

 to develop later. The most important of these latent organic 

 elements are those which, in many animals such as the Rotifers, 

 Daphnia, and various small insects, enter into the formation 

 of the reproductive elements. They may become isolated from 

 the very first segmentation, and thus constitute true 

 blastomeres. It is because his conclusions were based on these 

 specialized examples, which he took as representative of the 



