CHAP.XXXVIIl.] 



OVUM. 



553 



which is attached to the external clear membrane, or zona pellucida, 

 of the ovum. This layer is termed by Dr. Barry, the tunica granu- 

 losa. The so called retinacula are composed of similar cells, of 

 which many are also found floating in the fluid of the follicle, 

 which is entirely lined by them. 



Ovum. The ovum is invested with a clear, homogeneous, perfectly 

 transparent, firm, elastic, and tolef- Fig. 264. 



ably thick membrane, exhibiting an ^ 



appearance, when examined by the 

 microscope, very similar to that of 

 the elastic laminae of the cornea, 

 the vitellineoryolk membrane, or zona 

 pellucida ; the latter being the term 

 always employed in speaking of the 

 mammalian ovum. The zona pellu- 

 cida appears as a perfectly clear ring, 

 limited on either side by a well de- 

 fined dark outline. Within this mem- 

 brane is the yolk, which is composed 

 of a fluid containing proper yolk 

 granules and oil particles, with the 

 clear bright germinal vesicle, contain- 

 ing within it the germinal spot, lying 

 close beneath the zona pellucida. 



The ovum is about f^th, and the 



germinal Spot about fTwjioth. tO -g-^g-tll Mammalian ova. The upper figure shows 



. . ... an ovum at an early stage of development. 



Of an inch in diameter. The second figure, a mature ovum. a. Zona. 



,, , T i pellucida. b. Yolk. c. Germinal vesicle, d. 



The yolk granules dltier mUCh in Germinal spot. The lower figure shows the 



\ T i zona pellucida a, ruptured, and the escape of 



Size and form in aitierent animals, the yolk granules (A) and germinal vesicle 



, through the opening. From Costa. 



They are much more numerous in 



Fig. 265. 





Ova in various stages of development, from the toad's ovary, 

 are observed. 



VOL. IT. 



In the right figure some very small ones 



O O 



