THE EAKLY DEVELOPMENT OF THE MAKSUPIALIA. 39 



is hardly^ if at all, detectable, so that one lias to depend 

 partly on the relative thickness of the cells, partly, and, 

 indeed, mainly, on the yolk-body in determining which 

 hemisphere is which. 



The blastocyst cavity is tensely filled by a coagnlable fluid 

 derived fi-om that poured into the uterine lumen through the 

 secretory activity of the uterine glands. Also situated in the 

 blastocyst cavity, in contact with the inner surface of the 

 wall in the region of the upper pole, is the spherical yolk- 

 body (fig. 29, y.h.). It becomes ovei'grown and enclosed in 

 the blastocyst cavity as the result of the completion of the 

 cellular wall over the upper polar region, much in the same 

 sort of way as the yolk in the meroblastic egg becomes 

 enclosed by the peripheral growth of the blastoderm. In the 

 majority of my sections of early blastocysts the yolk-body 

 has been dragged away from contact with the formative cells 

 through the coagulation of the albuminous blastocystic fluid, 

 and lies more or less remote from the wall enclosed by the 

 coagulum, except on the side next the upper hemisphere (fig. 

 31, y.b., e.g.). In two instances, one of which is shown in 

 fig. 32, 1 find the yolk-body had become so firmly attached to 

 one of the formative cells that the coagulum formed during 

 fixation failed to detach it, and only succeeded in drawing it 

 out to a pear-shape. 



The yolk-body, it may here be mentioned, persists for a 

 considerable time in the blastocyst cavity; I have found it 

 shrunken indeed, but still recognisable, in relation to the 

 embryonal area in vesicles 4'5-6 mm. in diameter. And 

 there may even appear within it peripherally, irregular strands 

 which stain deeply with iron-htematoxylin and which recall 

 those forming the peripheral deutoplasmic network of the early 

 blastomeres. Eventually, however, it seems to disappear, its 

 substance passing into the blastocystic fluid, so that, as already 

 remarked, it fulfils in this indirect way its original destiny. 



Normally the cavity of the just completed blastocyst con- 

 tains no cellular elements whatever. In one otherwise 

 perfectly normal blastocyst (-39 mm. diam.) I find present, 



