1 8 GENERAL HISTORY. 



nuclear mass of protoplasm the latter structure may be called a 

 germogen. 



In some cases the whole of the germinal cells eventually 

 become ova, but as a rule only a small proportion of them have 

 this fate, the remainder undergoing various changes to be spoken 

 of in the sequel. 



Extended investigations have shewn that the distinction 

 between germinal cells which are independent cells from the 

 first, or derived from a germogen in which the nucleated proto- 

 plasm is not divided into cells, is an unimportant one ; and 

 closely allied forms may differ in this respect. It is moreover 

 probable that a germogen of nucleated protoplasm is less com- 

 mon than is often supposed : it being a matter of great difficulty 

 to determine the structure of the organs usually so described. 

 A germogen is stated to be found in most Platyelminthes, 

 Nematoidea, Discophora, Insecta, and Crustacea. 



A more important distinction in the origin of the germinal 

 cells is that afforded by their position. In this respect three 

 groups may be distinguished, (i) The germinal cells may form 

 the lining of a sack or tube, having the form of a syncytium or 

 of an epithelium of separate cells (Platyelminthes, Mollusca, Ro- 

 tifera, Echinodermata, Nematoidea, Arthropoda). (2) Or they 

 may form a specialized part of the epithelium lining the general 

 body cavity (Chaetopoda, Gephyrea, Vertebrata). (3) Or they 

 may form a mass placed between the two elsewhere contiguous 

 primitive germinal layers (Ccelenterata 1 ). 



Types of transition between the first and second group are 

 not uncommon. Such types, properly belonging to the second 

 group, originate by a special membranous sack continuous with 

 the oviduct being formed round the primitively free patch of 

 germinal cells. Examples of this are afforded by the Discophora, 

 the Teleostei, etc. It is very probable that all the cases which 

 fall under the first heading may have been derived from types 

 which belonged to the second group. 



The mode of conversion of the germinal cells into ova is 

 somewhat diverse. Before the change takes place the germinal 



1 In all the Metazoa the generative organs are placed between the primitive 

 germinal layers ; and the peculiarity of their position in the Coelenterata depends on 

 the absence of a body cavity and of a distinct mesoblast. 



