38 ,1. p. HILL. 



lower or uuu-fonnative cell-riug similarly spread towards the 

 lower pole. As the blastomeres divide and spread tliey 

 become smaller and more flattened, and gradnally cohere 

 together, and so in this way they eventnally give origin to a 

 complete nnilaminar layer lining the inner surface of the 

 sphere formed by the egg-envelopes. It is this nnilaminar 

 layer which constitutes tlie wall of tiie blastocyst. 



The just completed blastocyst of Dasyurus is !i spherical 

 fluid-filled vesicle measuring about '4 mm. in diameter (PI. 3, 

 figs. 27-29, PI. 6, figs. 61, 62), and invested externally by 

 the thin zona and the shell-membrane (■0075-*0078 mm. in 

 thickness). 'J'he albuir.en layer has completely disappeared, 

 and the shell-membrane, zona, and cellular wall are from 

 without inwards in intimate apposition. The smallest com- 

 plete vesicles which I have examined measure "oO mm. in 

 diameter (figs. 27, 61), and in one of these I find the cellular 

 wall consists approxitnately of about 108 cells. In four other 

 eggs of the same diameter and from the same female the wall 

 of the blastocyst is as yet incomplete at the lower pole (fig. 

 31, l.p.), and in these, rough counts of the cells yielded the 

 following respective numbers — 89, 93, 121, 128. In another 

 also incomplete blastocyst of the same batch, "41 mm. in 

 diameter (fig. 32), the cellular wall consists of about 130 cells. 

 The largest complete blastocyst in this same batch measured 

 •49 mm. in diameter, so that we have a range of variation in size 

 of the just completed blastocyst extending from "39 to "49 mm. 



The unilaminar wall of the blastocj'st consists of a con- 

 tinuous layer of more or less flattened polygonal cells (figs. 

 27-29, 61, 62) lying in intimate contact with the zona, itself 

 closely applied to the shell-membrane. Over the lower hemi- 

 sphere the non-formative cells are on the whole larger and 

 plumper than the formative cells of the upper hemisphere, 

 and in surface examination they appear somewhat denser 

 owing to the fact that they possess much more marked peri- 

 nuclear zones of dense cytoplasm than do the formative cells 

 (cf. fig. 63, representing a -6 mm. vesicle). In sections, 

 however, this latter difference is much less obvious, indeed, 



