34 Introduction to Animal Morphology. 



CHAPTER VI. 



REPRODUCTION. 



The organs hitherto referred to deal with the well- 

 being of the individual : for the propagation of the 

 species other apparatuses are needed. The simplest 

 morphological individuals reproduce by discontinuous 

 fission or gemmation. Higher forms show a more 

 complex mode, which, however, still partakes more or 

 less obviously of these primitive methods. In this 

 process two cells co-operate : one organ derived from 

 the endoderm or inner germ layer of the embryo* (the 

 ovary), forms, by gemmation of its component plastides, 

 special cells (ova), which are to be the new individuals. 

 Another often similar organ, but derived from the 

 ectoderm or outer germ layer* (testis), forms elements 

 also from its own plastides, whose function is to unite 

 with and stimulate the ova into development (sperma- 

 tozoa). What the nature of the stimulation is we 

 know not, and it is as yet idle to theorize, though the 

 diverse origin of the two elements is suggestive. In 

 the simplest cases the organs producing these diverse 

 elements are, though dissimilar in origin, side by side 

 or united, or we may find them at separate parts of 

 the same animal. The ova are nucleated cells; the 

 spermatozoa are nucleated flagellate plastides formed 

 within a mother-cell. If ovaria and testes co-exist in 

 one individual, it is called monooecious or hermaphro- 

 dite. If the sex organs are in separate individuals 

 they are called dioecious. The contents of the sex 

 organs may be emitted by simple rupture (dehiscence), 



* E, Van Beneden, 



