42 J, r. HILL. 



Selenka's Urentodermzelle, — Whilst the 42- and Q8- 

 celled blastocysts described by Selenka may be regarded 

 as normal so far as the occurrence of polar openings and 

 the constitution of their wall are concerned, I hold them to be 

 abnormal in respect of the presence in each of a single large 

 yolk-laden cell, regarded by Selenka as entodermal in signifi- 

 cance. It is well to point out that Selenka was not able 

 actually to determine the fate of this cell; he merely presumed 

 that it took part in tlie formation of the definitive entoderm. 

 No such cell occurs in normal blastocysts of Dasyurns at any 

 stage of developmeut, and in my opinion Selenka's "urento- 

 dermzelle " is none other than a retarded and displaced 

 blastomere, i.e. a blastomere which has failed for some 

 reason to divide, and which has become secondarily enclosed 

 by the products of division of its fellows, and I am 

 strengthened in this interpretation by the occurrence in 

 an abnormal blastocyst of Dasyurus of just such a large 

 cell as that observed by Selenka. The vesicle in question 

 is one of the batch already referred to, and measured '397 mm. 

 in diameter. The cellular wall (fig. o7) is apparently normal, 

 but is incomplete at one spot, and the gap so left is occupied 

 by a large binucleated cell, rich in deutoplasin and measuring 

 "12 X *072 mm. (fig. 37, ahu.). This cell corresponds in its 

 size and cytoplasmic characters with a non-formative blasto- 

 mere of about the IG-celled stage, and I regard it simply as 

 a blastomere which has failed to undergo normal division. 

 In another abnormal blastocyst ("39 mm. diam.) from the 

 same batch, the cellular wall appears complete and normal, 

 but the blastocyst cavity contains a group of about sixteen 

 spherical cells averaging about "032 mm. in diameter, and in 

 yet another abnormal egg of the same diameter and batch 

 there is present an incomplete layer of flattened cells over 

 one hemisphere, and towards the opposite pole of the egg- 

 sphere there occurs a group of spherical cells of variable size 

 and some of them multinucleate. In this aVjnornial egg it 

 appears as if the formative cells had divided in fairly normal 

 fashion, whilst the non-formative cells had failed to do so. 



