190 RELATIONS OF OVA AND GEMMAE. 



VIII. 



RELATIONS OF OVA AND GEMM^. 



§ 1. The arguments which have now been advanced for 

 establishing a homology between the sexual zooids of some 

 alternating species, and the more ordinary form of repro- 

 ductive organs, have proceeded on the assumption that a 

 difference between gemmse and ova, or corresponding bodies, 

 may in all cases be clearly ascertained, however much they 

 may resemble each other in general appearance ; and that 

 ill the latter we have a satisfactory basis from which to 

 estimate the relative import of other structures. So much 

 has indeed been generally admitted by naturalists, for it has 

 been commonly held that these bodies may be recognized 

 both by the necessity of impregnation for their farther de- 

 velopment, and by certain peculiarities of structure. 



It is allowed that these peculiarities are not always ap- 

 parent at first. Thus it has been remarked — especially in 

 orders noted for a proliferous tendency, as the Polyzoa and 

 Polypifera — that the nascent ovum bears a close resem- 

 blance to an ordinary gemma, both in itself and in its place 

 of origin, and other relations to the surrounding tissues. 

 When the development, however, is complete, the most re- 

 cent observations go to show that in Animals a true ovum 

 always contains in its interior a characteristic nucleated ger- 

 minal vesicle, and that in Vegetables the germ, (from which 

 after fertilization by the spermatic particles the embryo is 

 formed), though differing widely in different cases — as in 

 cryptogamic and phanerogamic plants — yet admits of having 



