46 J. p. HILL. 



•0026 mm. The zona is now no longei* recognisable as an 

 independent membrane. In blastocysts of this stage of 

 growth a variable nunaber of small spherical cells or cell- 

 fragments are frequently met with in the blastocyst cavity, 

 usually lying in contact with the inner aspect of the cellular 

 wall (fig. 34, i.e.). In some blastocysts such structures are 

 absent, in others one or two may be present, in yet others 

 numbers of them may occur. They may be definitely nucleated, 

 but this is exceptional ; more usually they contain one or more 

 deeply staining granules (of chromatin ?), or are devoid of 

 such. They are of no morphological importance, and I think 

 there can be no doubt that they represent cells or fragments 

 of cells which have been separated off from the cellular wall 

 during the process of active growth. They are of common 

 occurrence in later blastocysts, and it is possible the so-called 

 " yolk-balls " observed by Selenka in Didelphys are of the 

 same nature. 



If we pass now to vesicles from 1 to 3 or 3*5 mm. in 

 diameter, Ave find tlie wall still unilarainar, but considerably 

 more attenuated than it is in the blastocysts last referred to. 

 In a vesicle with a diameter of 1*24 mm. the shell-membrane 

 has a thickness of about 'OOlo mm., whilst the cellular wall 

 has a thickness of only •004o mm. In a 3'5 mm. vesicle the 

 shell-membrane measures about '0012 mm., whilst the cellular 

 wall ranges from -0018 to '0048 mm. in thickness. A small 

 portion of the wall of a vesicle, 2*4 mm. in diameter, is shown 

 in PI. 6, fig. 64. In these later vesicles I have failed to detect, 

 either in surface examination of the vesicles in toto or in 

 sections, any regional differences between the cells indicative 

 of a differentiation of the wall into upper or formative, and 

 lower or non-formative, hemispheres. Everywhere the wall 

 is composed of flattened, extremely attenuated cells, polygonal 

 in surface view, and all apparently of the same character. It 

 might therefore be supposed that the polarity, which is recog- 

 nisable in early blastocysts, and which is dependent on the 

 pronounced differences existent between the cells of the 

 upper and lower rings of the 16-celled stage, is of no funda- 



