8 EMBRYOLOGY. 



the process of conjugation. Since it is a question of fundamental 

 importance to determine how sexual reproduction 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 con- 

 jugation 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 perhaps 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 aggregation. It results from this that the segmenta- 

 tion 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 permissible 

 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 occurrence, 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 without 

 previous conjugation is perhaps provided against in the extrusion of 

 the so-called 'directive body'. 



With the differentiation of special germinal cells, to take the place 

 of the whole individual in the act of conjugation, the possibility of 

 each act of conjugation resulting in the production of only a single 

 organism became introduced. Germinal cells can be indefinitely pro- 

 duced, and the reproductive capacity of a single individual is there- 

 fore unlimited ; while if two whole individuals conjugated and only 

 produced one from the process, the result would be a diminution 

 instead of an increase in the race 1 . 



1 In the vegetable kingdom there are numbers of types of Thallophytes, which throw 

 'A considerable amount of light on the relation between sexual reproduction and con- 

 jugation. Subjoined are a few of the more striking cases. In Pandorina at the time 

 of sexual reproduction the cells which constitute a colony divide each into sixteen, and 

 the products of their division are set free. Pairs of them then conjugate and perma- 

 nently fuse. After a resting stage the protoplasm is set free from its envelope after 



