POLYEMBRYONIC DEVELOPMENT IN TATUSIA 575 



There are, however, two distinct types of cells. . The bulk of the mass 

 consists of rather large, ill-defined cells, the cytoplasmic and nuclear por- 

 tions of which do not stain well. Scattered among these are smaller 

 cells, which are found much more abundantly hi that part of the mass 

 lying towards the cavity of the vesicle. The smaller cells are character- 

 ized by their sharply denned outlines and by the ease with which they 

 take the. stain. There is considerable evidence to indicate that the 

 smaller cells are gradually undergoing a segregation from their larger 

 fellows to form the hypoblastic layer of the vesicle. At any rate the 

 hypoblast upon its completion possesses cellular elements that very 

 closely simulate the smaller cells of the earlier stages. 3 



This statement was made before I had had an opportunity of 

 examining Hill's ('10) excellent paper on the early development of 

 the 'native cat/ Dasyurus viverrinus. In this paper Hill shows 

 in the clearest possible way that the entoderm arises in a manner 

 quite similar to that in the blastocyst of Tatusia, and I shall there- 

 fore briefly state his general results on the point under discussion. 

 It will be recalled that in the 16-cell stagfe the blastomeres are ar- 

 ranged in two superimposed rings of eight cells each. The eight 

 upper cells, which are smaller than the eight lower, are destined 

 to produce the formative or embryonal region of the blastocyst 

 wall, while the eight lower cells will give rise to the non-formative 

 or extra-embryonal region. In accordance with the characteristic 

 mode of development in the marsupial, no morula stage is formed 

 in the egg of Dasyurus, but the blastomeres proceed directly to 

 form the wall of the blastocyst. This is brought about through 

 the division of the blastomeres of each ring and their gradual 

 spreading toward the opposite poles, on contact with the inner 

 surface of the sphere formed by the zona and the shell-membrane. 

 The daughter blastomeres continue to divide and eventually pro- 

 duce a complete cellular lining to the zona sphere, constituting 

 the unilaminar wall of the blastocyst. The wall remains in this 

 unilaminar condition until the blastocyst attains a diameter of 

 4 to 5 mm. 



Hill here draws the most fundamental conclusions of his paper, 

 pointing out that the formative or embryonal region, which from 

 the first possesses no covering of trophoblast (i.e., Rauber's layer), 



3 Loc. cit., pp. 369-370. 



