INTRODUCTION. II 



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 pos- 

 sibility of each act of conjugation resulting in the production of 

 only a single organism became introduced. Germinal cells can 

 be indefinitely produced, and the reproductive capacity of a 

 single individual is therefore 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 . 



It must be admitted that, in the present state of our know- 

 ledge, the passage from reproduction by spores following con- 

 jugation, to true sexual reproduction, can only be traced in a 

 very speculative manner, and that a further advance in our 

 knowledge may prove that the steps which I have attempted to 

 sketch out are far from representing the true origin of sexual 

 differentiation. The peculiar conjugation and fusion of two 

 individuals to form Diplozoon paradoxum may be alluded to in 

 this connection. This fusion merely results in the attainment 

 of sexual maturity by the two conjugating individuals. It does 

 not appear to me probable that this conjugation is in any way 

 connected with the conjugation of the Protozoa, but the reverse 

 must be borne in mind as a possibility. 



It is not easy to decide whether the hermaphrodite or the 



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

 throw a considerable amount of light on the relation between sexual reproduction and 

 conjugation. 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 

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

 after division into two or four parts. Each of these then divides into sixteen coherent 

 cells and constitutes a new Pandorina colony. In (Edogonium the fertilization is 

 effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere 

 (oospore) then undergoes segmentation like the ovum of an animal ; but the segments, 

 instead of uniting to form a single organism, separate from each other, and each of 

 them gives rise to a fresh individual (swarm-spore) which grows into a perfect CEdo- 

 gonium. In Coleochcete the impregnation and segmentation take place nearly as in 

 CEdogonium, but the segments remain united together, acquire definite cell walls, and 

 form a single embryo. There is in fact in Coleocheete a true sexual reproduction of 

 the ordinary type. (Vide S. H. Vines "On alternation of generation in the Thallo- 

 phytes." Journal of Botany, Nov., 1879.) 



