THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 37 



2. Formation of the Blastocyst. 



It is characteristic of the Marsupial that the cleavage-cells 

 proceed directly to form the wall of the blastocyst, without 

 the iuterventiou of a morula stage, as in the Eutheria. 



The fifth cleavages are meridional, each of the eight cells 

 of the two rings of the 16-celled stage becoming subdivided 

 vertically into two, so that there results a 32-celled stage 

 consisting of two rings, each composed of sixteen cells. As 

 might be expected, the smaller less yolk-rich cells of the 

 upper ring tend to divide more rapidly than the larger yolk- 

 laden cells of the lower ring, but the difference in the rate of 

 division of the two is only slight. I have, for example, 

 sections of a 17-celled stage (that already i-eferred to, fig. 26) 

 consisting of nine formative cells (= 6 + [1 x 2] + 1 in 

 division) and eight non-formative cells, and also of a 31 -celled 

 stage (PI. 6, tig. 59, seen from lower pole; cf. also tig. 60, 

 showing a side view of another 31-celled egg, both eggs 

 •375 mm. in diameter), consisting of sixteen formative and 

 fifteen non-formative cells, of Avhich one is in process of 

 division. But I have also preparations of 32-celled eggs with 

 an equal number of formative and non-formative cells, 

 showing that the latter may make up their leeway, the former 

 resting meantime. On the other hand, the cells of the two 

 rings may divide more irregularly, as evidenced by a stage 

 of about forty-two cells, consisting approximately of twenty- 

 three formative cells ( = 9 + [7 x 2J ) and nineteen non- 

 formative (= 13 + [3 X 2]). Whatever the rate of division, 

 the important point is that the division planes are always 

 radial to the surface, so that all the resulting blasto- 

 meres retain a superticial position in contact with the inner 

 surface of the supporting sphere formed by the zona and 

 shell-membraue. In apposition with the continuous surface 

 afforded by that, the blastomeres, continuing to divide, 

 gradually spread round towards the poles, the descendants of 

 the upper or formative cell-ring gradually extending towards 

 the upper pole marked by the yolk-body, whilst those of the 



