INTRODUCTION. 9 



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

 the passage from reproduction by spores following conjugation, 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 repre- 

 senting the true origin of sexual differentiation. The peculiar con- 

 jugation 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 dioecious 

 state is the primitive one, or in other words whether the two con- 

 jugating cells, from which I have supposed the sexual products to 

 originate, were derived in the first instance from one or from two 

 colonies of Protozoa. On purely a priori grounds it seems probable that 

 they were originally formed in one colony, and that their derivation from 

 two colonies or individuals was inaugurated when the spermatozoon 

 became motile. There can be no doubt that the dioecious state is a 

 very early one, and that the majority of existing cases of hermaphro- 

 ditism are secondary. 



The above considerations with reference to the male and female 

 cells appear to indicate that they were primitively homodynamous ; a 

 conclusion which is on the whole borne out by the history of their 

 development. 



Although the modes of reproduction amongst the Metazoa have 

 been divided into the classes sexual and asexual, there is nevertheless 

 one mode of asexual reproduction which ought to be classified with 

 the sexual rather than with the asexual modes. I mean partheno- 

 genesis, which consists essentially in the development of the ovum 

 into a fresh individual without previous coalescence with the male 

 element. This mode of reproduction, which has a very limited 

 range in the animal kingdom, being confined to the Arthropoda 

 and Rotifera, is undoubtedly secondarily derived from sexual repro- 

 duction. The conditions of its occurrence are discussed in the second 

 chapter. 



It is remarkable that in certain cases the absence of fertilization 

 causes the production of males (Bees, a Saw-fly, Nematus ventricosus, 



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

 and constitutes a new Paudoiina colony. In (Eclogoninm the fertilization is effected 

 by a spermatozoon fusing with an oo. sphere (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 CEdogouium. In, 

 Coleochfete the impregnation and segmentation take place nearly as in (Edogo- 

 nium, but the segments remain united together, acquire definite cell walls, and form a 

 single embryo There is in fact in Celeoch;ete a true sexual reproduction of the ordinary 

 type. (Vide S. H. Vines " On alternation of generation in the Thallophytes." Journal 

 of Botany, Nov., 1870.) 



