THE OVUM. 



FIG. 2. OVUM OF 

 CARMARINA (GERYO- 



NIA) HASTATA. (Copied 



from Haeckel.) 



gd. Body of ovum. 

 gv. Germinal vesicle. 

 gni. Germinal spot. 



cells frequently multiply by division. The 

 change itself usually involves a considerable 

 enlargement of the germinal cell, and gene- 

 rally a change in the character of the ger- 

 minal vesicle, which in most young ova 

 (fig. 2) is very large as compared to the 

 body of the ovum. The most complicated 

 history of this kind is that of the ovum of 

 the Craniata. (Vide pp. 56, 57.) 



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 struc- 

 ture. 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. 



The ovum is of course nourished like 

 every other cell by the nutritive fluids in 

 which it is surrounded, and special provi- 

 sions are made for this, in that the ovary is 

 very frequently placed in contiguity with 

 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 Hy- 

 drozoa (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 in- 

 vested by a special layer of cells, which 

 then constitutes what is known as a fol- 

 licle. The cells which form the follicle are 

 often germinal cells, e.g. Holothuria, Insecta (fig. 17), Vertebrata 



2 2 



FIG. 3. FEMALE 



GONOPHORE OF TUBU- 

 LARIA MESEMBRYAN- 



THEMUM. CONTAINING 



ONE LARGE OVUM (ov) 

 AND A NUMBER OF GER- 

 MINAL CELLS (g.C.). 



ep. Epiblast (Ecto- 

 derm). Ay. Hypoblast 

 (Entoderm). ov. Ovum. 

 g.c. Germinal cells. 



