THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 59 



whole cells of the type indicated by the ratio 6 : 1 predominate, 

 and we should hardly expect to find such cells dividing tangen- 

 tially. In fact, the only undoubted examples of such division I 

 have met with occur in the single abnormal vesicle present in 

 the '04 batch. In this particular vesicle, which had a diameter 

 of 3 mm. and was thus smaller than the others, there was 

 present on what appeared to correspond to the formative 

 hemisphei-e of the normal blastocyst a well-defined and con- 

 spicuous ovalish patch, 1'23 x '99 mm. in diameter.i Sections 

 show that over this area the cells of the uuilaminar wall are 

 much enlarged and more or less cubical in form, their thick- 

 ness varying from "012 to "019 mm. These cubical cells 

 exhibit distinct evidence of tangential division, both past and 

 in progress. But in normal vesicles, whilst mitotic figures are 

 quite commonly met with in the cells of the formative region 

 (in which, indeed, they are more numerous than in those of 

 the non-formative region), I have failed to find in my sections 

 after long-continued searching even a single spindle disposed 

 directly at right angles to the shell-membrane ; the mitotic 

 spindles lie disposed either tangentially to the surface or 

 obliquely thereto. 



For the determination of the mode of origin of the 

 primitive entodermal cells, it is absolutely necessary to 

 study both in toto preparations of the formative region, 

 i.e. small portions of the uuilaminar wall stained and 

 mounted on the flat, and sections of the same. Sections alone 

 are, on the whole, distinctly disappointing so far as the 

 question under discussion is concerned, and, indeed, give one 

 an altogether inadequate idea of the primitive entodermal cells 

 themselves, seeing that practically all one can make out is that 



^ Curiously enough, amongst the '99 vesicles there also occuiTed 

 a single small one, likewise 3 mm. in diameter, and with a thickened 

 patch 1"28 X 1 mm. in diam^eter, quite similar in its character to that 

 described in the text. I am as yet uncertain whether the thickened 

 area in these two vesicles represents the whole of the formative hemi- 

 sphere of normal blastocysts or only a hypertrophied part of the same, 

 or whether, indeed, it may not represent the retarded non-formative 

 hemisphere. 



