432 



Mr. A. Fraser. On the Inversion of the [Dec. 21, 



a rat pregnant six days and twelve honrs. This ovum has been cut 

 parallel to its long axis, which lies in the transverse vertical axis of the 

 uterus. (Fig. 1.) 



Fig. 1. 



Section of ovum from rat, 6 days and 12 hours. 



The ovum measures in its long axis 0*16 millim., in breadth 0"06 

 millim. 



Of the zona there is no trace. The ovum has reached the stage of a 

 blastodermic vesicle, B.v., the wall of which is formed over the greater 

 part of its circumference by a delicate membrane, in which there lie 

 sparsely scattered cells. At the placental pole of the ovum, or 

 at that part of the circumference turned to the mesometrial side of 

 the uterus, there projects into the cavity of the vesicle a flask-shaped 

 mass of cells, the long axis of which lies in the same direction as the 

 long axis of the vesicle, and measures 0*09 millim. ; in breadth it 

 measures 0"04 millim. 



It will thus be seen that this mass of cells fills up the greater part 

 of the cavity of the vesicle, and the placental pole of the mass is 

 continuous with, and forms part of, the wall of the blastodermic 

 vesicle. The flask-shaped mass of cells inside the vesicle, and which 

 has its origin at the placental pole, is formed of cells, the subsequent 

 history of which shows that they are mainly epiblastic with a cover- 

 ing of hypoblast. 



The epiblast cells, e.p., are roughly circular, approaching the columnar 

 character in form, and at the placental pole of the ovum lie in contact, 

 if not actually continuous, with this limited area of the wall of the 

 blastodermic vesicle. They are covered on the side next the cavity 

 of the blastodermic vesicle by a single layer of hypoblast cells, hy., 

 which also at the placental pole of the ovum lie contiguous with the 

 wall of the blastodermic vesicle at the margin of the epiblast cells. 



The vesicle, then, at its placental pole exactly resembles the ger- 

 minal area of an ordinary vesicle, such as that of the rabbit, which, 

 taking the widely accepted description, is formed of a protective cover- 

 ing of cells (Rauber's layer of " Deckzellen") continuous with the wall 



