46 . CARL GOTTFRIED HARTMAN 



of the entoderm, as suggested by Selenka. To suppose that the 

 cells in various parts of the blastocyst cavity migrate to the 

 formative pole of the egg, attach and proliferate as definitive 

 entoderm of the blastocyst is asking too much of them ; nor does 

 this presumption accord With facts observed in studying the 

 later stages. 



It will be recalled that Selenka ('87) observed included cells 

 in the 20-celled egg and in the two young blastocysts studied by 

 him. Inasmuch as Hill ('10) criticised these stages as abnormal, 

 and as the matter is one of importance, it will now be discussed 

 in detail. 



b. Cells included in the blastocyst cavity 



Of the included cells above mentioned Hill says: 



Whilst the 42- and 68-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 significance. 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 the formation of the defini- 

 tive entoderm. No such cell occurs in normal blastocysts of Dasyurus 

 at any stage of development, and in my opinion Selenka's 'urentoderm- 

 zelle' is no other than a retarded and displaced blastomere. . . ... 



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 This cell corresponds in its size 



and cytoplasmic characters with a non-formative blastomere of about 

 the 16-celled stage, and I regard it simply as a blastomere which has 

 failed to undergo normal division.^ 



In two other incomplete blastocysts of less than 0.4 mm. 

 diameter Hill found abnormal included cells; but in later blasto- 

 cysts 0.5 to 0.8 mm. in diameter, taken from three different 

 females, he often finds included cells or fragments thereof. To 

 quote again: 



In blastocysts of this stage of growth a variable number of small 

 spherical cells or cell-fragments are frequently met with in the blasto- 

 cyst cavity, usually lying in contact with the inner aspect of the cellu- 

 lar wall. In some blastocysts such structures are absent, in others 



9 Loc. cit., p. 42. 



