THE DEVELOPMENT OP THB OVUM. 113 



Section XXU.The Development of the Ovum. 

 PLATE X., Fias. 8 TO 14. 



The ovum in the earliest stage of development in which I 

 have yet succeeded in observing it, is ovate in form, and mea- 

 sures about l-300th of an inch in its long diameter. It 

 consists of a single cell, the germinal vesicle, which occu- 

 pies the greater part of its interior, and is surrounded by a 

 thin layer of clear fluid. This is covered by an external en- 

 velope of minute cells the yolk sac 1-30 00th of an inch in 

 diameter, loosely adherent to each other, and contained in an 

 extremely delicate membrane the ovi-capsule. These cells 

 refract light strongly, and make it very difficult to observe the 

 germinal vesicle. 



The ovum soon after becomes globular; the external cells 

 adhere firmly to each other, and become prismatic by pressure ; 

 and the yolk, instead of containing a single vesicle, consists of 

 six to eight cells, which fill the whole interior of the yolk sac. 

 I am doubtful whether this multiplication of cells is analogous 

 to yolk-segmentation in vertebrates, but I suspect that it is. 



The ovum now enlarges rapidly, assumes an ovate form, and 

 the yolk cells become spheroidal, multiplying until about 

 thirty-two are contained within the ovum (Plate X., fig. 8 c), 

 after which they degenerate into a homogeneous molecular 

 mass. A clear space is then gradually formed at the upper 

 part of the yolk, which is the representative of the germinal 

 disc in vertebrates. 



The changes which take place in the yolk cells are very remarkable : at 

 first they exhibit a cell wall and a large nucleus, but after a time the 



