SEGMENTATION OF THE OVUM OF THE SHEEP. 215 



In tliis specimen tlie strands of protoplasm seen in the former 

 are less numerous. The cavity is rather larger (fig. 14). The 

 light cells are not so numerous nor so well differentiated as in 

 the former case; they form an irregular mass, partially sur- 

 rounded by the darker cells. I cannot be sure that there are 

 more than eight or nine of the lighter cells. The dark cells 

 number about seventy-five. It is, however, very difficult to 

 count them. 



Fig. 15 is a drawing from a section of a rather more ad- 

 vanced specimen. In this the cavity of blastocyst is well 

 developed, and the connecting strands of protoplasm are only 

 found in the narrow space round the embryonic pole. 



There is now very little difference in colour between the 

 several segments. I think my figure represents very fairly 

 accurately such difi'erence as there is. This is a very slightly 

 lighter colour of the segments lying between the outer wall 

 and the innermost layer of the inner mass. 



The embryo now fills the whole of the space inside the zona 

 radiata, the thickness of which has not materially altered since 

 it left the ovary. There has been no additional investment. 



From this moment the internal pressure increases rapidly, 

 and the vesicle expands, and causes the zona radiata to become 

 very much attenuated. Such is the condition of my next 

 embryo, which is shown in fig. 16. This embryo was probably 

 eight days old ; the age was not noted. The embryo formed a 

 spherical, transparent, perfectly typical mammalian blasto- 

 dermic vesicle. It differs from the foregoing specimen in the 

 great attenuation of the zona radiata and outer layer of cells, 

 and in a general reduction in the size of the inner mass. 



It is now impossible to distinguish any lighter coloured 

 area extending over more than a very limited space. There 

 are no visible boundaries to the cells. I am not sure that the 

 outer layer is distinct from the inner mass. 



Fig. 17 has been drawn from a section of a specimen taken 

 from the same uterus, but not the same side as the preceding 

 specimen. It is undoubtedly slightly more advanced. The 

 zona radiata had ruptured or had been absorbed, and the 



