10 EMBRYOLOGY. 



themselves up into an organism resembling one of the parents. 

 The sexual process has obviously at first sight a very close 

 resemblance to the process of conjugation. Since it is a ques- 

 tion of fundamental importance to determine how sexual repro- 

 duction originated, it becomes necessary to examine how far 

 this apparent resemblance is a real one, and how far sexual 

 reproduction can be derived from reproduction following upon 

 conjugation. 



In spite of the general similarity between the two processes 

 there is an obvious difficulty in comparing them, in that the 

 result of conjugation is usually the breaking up of the individual 

 formed by the fusion of two other individuals into a number of 

 new organisms, while the result of the fusion which takes place 

 in sexual reproduction is the formation of a single new organism. 

 This difference between the two processes, great as it is, is per- 

 haps apparent rather than real. It must be remembered that a 

 single individual Metazoon is equivalent to a number of Protozoa 

 coalesced to form a single organism in a higher state of aggre- 

 gation. It results from this that the segmentation of the ovum 

 which follows the sexual act may be compared to the breaking 

 up of the product of conjugation into spores, the difference 

 between the two processes consisting in the fact that in the one 

 case the spores separate each to form an independent organism, 

 while in the other they remain united and give rise to a single 

 compound organism. 



If the above considerations are well founded it seems permis- 

 sible to accept the general view according to which sexual 

 reproduction is derived from conjugation. It is necessary to 

 suppose that, in a colony of Protozoa in the course of becoming 

 a Metazoon, the capacity of reproduction by spores became 

 localized in certain definite cells, and although the formation of 

 spores from these cells may have been possible without previous 

 conjugation, yet that conjugation gradually became established 

 as the rule. The differentiation of primitively similar conjugating 

 cells into male and female cells was probably a very early occur- 

 rence, since indications of an analogous differentiation, as has 

 already been mentioned, are found in certain existing Protozoa 

 (Monads, Vorticella, etc.). I have attempted to shew in the 

 second chapter that the breaking up of the cell into spores 



