THE OVUM. 15 



germinal cells which are independent cells from the first, or derived 

 from a germogen in which the nucleated protoplasm 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 common 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. (1) 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, Rotifera, Echinoderrnata, 

 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 (Coelenterata 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 cells frequently 

 multiply by division. The change itself usually 

 involves a considerable enlargement of the 

 germinal cell, and generally a change in the 

 character of the germinal vesicle, which in most HASTATA. (Copied from 

 young ova (fig. 2) is very large as compared to Haeckel.) 

 the body of the ovum. The most complicated ^te 

 history of this kind is that of the ovum of the gm . Germinal spot. 

 Craniata. (Vide pp. 45, 46) 



The ovum in its young condition is obviously nothing but a simple 

 cell ; and such it remains till the period when it attains maturity. 



Nevertheless the changes which it undergoes in the course of 

 its growth are of a very peculiar kind, and, consisting as they do 

 in many instances of the absorption of other cells, have led various 

 biologists to hold that the ovum is a compound structure. It 

 becomes therefore necessary to consider the processes by which the 

 growth and nutrition of the ovum is effected before dealing with the 

 structure of the ovum at all periods of its history. 



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

 layers; and the peculiarity of their position in the Crelenterata depends on the absence 

 of a body cavity and of a distinct mesoblast. 



