20 GENERAL HISTORY. 



(fig. 19). In other cases they seem rather to be adjoining con- 

 nective-tissue or epithelioid cells, though it is sometimes difficult 

 to draw the line between such cells and germinal cells. Ex- 

 amples of follicles formed of ordinary connective-tissue cells, 

 are supplied by Asterias, Bonellia (fig. 16), Cephalopoda (fig. 

 14), etc. 



A membrane enclosing the ovum without a lining of cells, as in many 

 Arachnida, vide p. 51, has no true analogy with a follicle and does not 

 deserve the same name. 



The function of the follicle cells appears to be, to elaborate 

 nutriment for the growth of the ovum. The follicle cells are not 

 as a rule directly absorbed into the body of the ovum, though 

 in some instances, as in Sepia (vide p. 40), they are eventually 

 assimilated in this way. 



In many cases some of the germinal cells form a follicle, 

 while other germinal cells form a mass within the follicle 

 destined eventually to be used as pabulum. Insects supply 

 the best known examples of this, but Piscicola, Bonellia (?) may 

 also be cited as examples of the same character. In the Cra- 

 niata (pp. 56 58) some of the germinal cells which advance a 

 certain distance on the road towards becoming ova, are even- 

 tually used as pabulum, before the formation of the follicle ; 

 while other germinal cells form at a later period the follicular 

 epithelium. A peculiar case is that of the Platyelminthes (fig. 9), 

 where a kind of follicle is constituted by the cells of a specially 

 differentiated part of the ovary, known as the yolk-gland. The 

 cells of this follicle may either remain distinct, and continue to 

 surround the ovum after its development has commenced, and 

 so be used as food by the embryo ; or they may secrete yolk 

 particles, which enter directly into the protoplasm of the ovum. 



For further variations in the mode of nutrition the reader is 

 referred to the special part of this chapter. Suffice it to say 

 that none of the known modes of nutrition indicate that the 

 ovum becomes a compound body any more than the fact of an 

 Amoeba feeding on another Amoeba would imply that the first 

 Amoeba ceased thereby to be a unicellular organism. 



The constitution of the ovum may be considered under three 

 heads : 



