958 OF GENERATION. 



after the name of its first discoverer ; but the more general and appropriate 

 designation of Ovisac has been given to it by Dr. Barry, who has shown that 

 it exists in other classes of Vertebrata. Between the Ovum and the Ovisac, in 

 Oviparous animals, there is scarcely any interval ; but in the Mammalia, a large 

 amount of granular matter (composed of nucleated cells, loosely aggregated toge- 

 ther) is present ; being especially found adherent to the lining of the jovisac, to 

 which it forms a sort of epithelium, or internal tunic, known as the membrana 

 granulosa; whilst it also forms a disk-like investment to the ovum, which is 

 termed the discus proligerus. The membrane which incloses the yelk in Mam- 

 malia has received, on account of its thickness and peculiar transparency, the 

 distinctive appellation of zona pellucida (Fig. 242, m v). The yolk, or vitellus 

 (j"), which is composed of albumen and oil-particles, with traces of cells, is 

 very small in the Mammalian ovum, its function being limited to the sustenance 

 of the germ during its earliest period of development ; and it corresponds rather 

 with that part of the yelk of the egg of the higher Ovipara which has been dis- 

 tinguished as the " germ-yelk/' in consequence of its direct participation in 

 the formation of the germinal substance than with that which has been 

 termed the " food-yelk," as not being incorporated with the germ, but being 

 destined for its subsequent nutrition by undergoing conversion into blood. 1 

 Occupying the centre of the vitelline mass, in the immature ovulum, is a pecu- 

 liar cell, very different in its aspect from the surrounding substance, which is 

 termed the germinal vesicle (Fig. 242, v <?) ; and this has a very distinct nucleus 

 (t g) known as the germinal spot. This cell must be considered as the essential 

 part of the ovum, and as homologous with the " germ-cell" or " embryonic vesicle" 

 of the Vegetable ovule. The Human Ovum is extremely minute ; not measur- 

 ing above l-120th of an inch in diameter, and being sometimes of no more than 



Fig. 242. 



Constituent parts of Mammalian Ovum : A, entire ; B, ruptured, with the contents escaping ; m v, vitelline 

 membrane ; j, yelk ; v g, germinal vesicle ; t g, germinal spot. 



half that size. The diameter of the germinal vesicle of the human ovum has 

 not yet been ascertained, owing to the difficulty of isolating it from the yelk ; in 

 the ovum of the rabbit, it is about l-720th of an inch ; and that of the ger- 

 minal spot, in the Mammalia generally, is from l-3600th to l-2400th of an inch. 

 963. It appears, from the researches of Valentin and Bischoff, that the 

 Graafian vesicle, or Ovisac, is formed previously to the Oyum, which is subse- 

 quently developed in its interior ; and it would seem that we may regard it as 



1 It has been recently maintained by Keinhardt that the Bird's egg is really homolo- 

 gous with the Graafian vesicle of the Mammal, and its entire contents ; the "food-yelk" 

 of the former being represented in the latter by the cellular substance surrounding the 

 zona pellucida, which is afterwards developed into the corpus luteum. 



