12 EMBRYOLOGY. 



dioecious state is the primitive one, or in other words whether 

 the two conjugating 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 d 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 herma- 

 phroditism 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 parthenogenesis, 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 reproduction. The 

 conditions of its occurrence are discussed in the second chapter. 



It is remarkable that in certain cases the absence of fertiliza- 

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

 ventricosus, etc.); more usually it results in the production of 

 females only, and there are very often in the Arthropoda a 

 series of successive generations of females all producing ova 

 which develope parthenogenetically into females; eventually 

 however, usually in direct or indirect connection with a change 

 of food or temperature, or other conditions, ova are formed 

 which give rise without fertilization both to males and females. 



The true asexual modes of reproduction amongst the Metazoa 

 consist of fission and gemmation. Gemmation is by far the most 

 widely disseminated of the two. Various as are the methods in 

 which it takes place, it seems nevertheless that cells derived from 

 all the germinal layers, and very frequently from all the im- 



