40 ■ J. p. jiiLi-. 



however, a small spheroidal body "028 mm. in diameter, 

 composed of glassy-looking cytoplasm enclosing a central 

 deeply staining granule. This I interpret as a cell or cell- 

 fi-agment which has been accidentally separated off from the 

 wall, and which has undergone degeneration. In later 

 blastocysts such cellular bodies exhibiting more or less 

 evident signs of degeneration are of fairly common occur- 

 rence. They are of no morphological significance. 



Selenka's ''Blastopore." — Normally the wall of the 

 blastocyst is first completed over the upper hemisphere, in 

 correspondence with the fact that the formative cells not 

 only divide somewhat more rapidly than the non-formative 

 but have a smaller extent of surface to cover, since the upper 

 cell-ring from which they are derived lies about midway 

 between the upper pole of the sphere formed by the egg- 

 envelopes and the equator of the same, whilst the lower cell- 

 ring from which the non-formative cells arise is approximately 

 equatorial in position. We thus meet with stages in the 

 lormation of the blastocystic wall such as are represented in 

 surface view on PI. 3, fig. 30, and in section in figs. 31 and 

 32, in which the blastocystic cavity, prior to the completion 

 of the cellular wall over the lower polar region, is more or less 

 widely open below. There can be no doubt, I think, but that 

 this opening corresponds to that observed by Selenka in his 

 42-celled " gastrula " of Didelphys and regarded by him as 

 the blastopore, since he believed the entoderm arose from its 

 lips. My observations conclusively show that it has no 

 connection whatever with the entoderm, this layer arising 

 from the formative region of the upper hemisphere, and that 

 it is a mere temporary opening of no morphological signifi- 

 cance, blastoporic or other. Prior to the completion of 

 the wall at the upper pole a corresponding opening is tem- 

 porarily present there also. Both owe their existence to the 

 characteristic way in which the blastocyst wall is formed by 

 the spreading of the products of division of the two cell-rings 

 of the 16-celled stage towards opposite poles in contact with 

 the surface provided by the enclosing egg-envelopes. 



