1G 



GENERAL HISTORY. 



The ovum is of course nourished like every 

 other cell by the nutritive fluids in which it 

 is surrounded, and special provisions are made 

 for this, in that the ovary is very frequently 

 placed in contiguity \vith vascular channels. 

 But in addition to such nutrition a further 

 nutrition, the details of which are given in the 

 special part of this chapter, is provided for in 

 the germinal cells which do not become ova. 



In the simplest case, as in many Hydrozoa, 

 (fig. 3), the germinal cells which do not become 

 ova are assimilated by the ovum much in the 

 manner of an Amoeba. 



In other cases the ovum becomes invested 

 by a special layer of cells, which then constitutes 

 what is known as a follicle. The cells which 

 form the follicle are often germinal cells, e.g. 

 Holothuria, Insecta (fig. 17), Vertebrata (fig. 19). 

 In other cases they seem rather to be adjoining 

 connective-tissue or epithelioid cells, though it is 

 sometimes difficult to draw the line between such 

 cells and germinal cells. Examples of follicles 

 formed of ordinary connective-tissue cells, are 

 , Bonellia (fig. 16), Cephalopoda (fig. 14), etc. 



FIG. 3. FEMALE 

 GONOPHOEE OF TUBU- 

 LAKIA MESEMBRTANTHE- 

 MUM. CONTAINING ONE 



LARGE OVUM (ov) AND A 

 NUMBER OF GERMINAL 

 CELLS (fJ.C.) 



ep. Epiblast (Ecto- 

 derm), liy. Hypoblast 

 (Eiitoderm). or. Ovum. 

 g.c. Germinal cells. 



supplied by Asterias 



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

 Araclinida, vide p. 41, Las no true analogy with a follicle and does not 

 deserve the same name. 



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

 ment 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. 32), 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 even- 

 tually 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 Craniata (p. 45, 46) some of the germinal 

 cells which advance a certain distance on the road towards becoming 

 ova, are eventually 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 



