32 J. P. HILL. 



Moreover, so far as tlie Ratheria are concerned, it affords us, 

 I believe, a striking and hitherto unrecognised example of a 

 phenomenon to which Lillie ('99) has directed attention, viz. 

 adaptation in cleavage. 



Fio-. 22 shows a horizontal section tlu'ough the 4-celIed 

 stage, and fig. 23 a vertical section of the same. Tlieblasto- 

 meres in their cytoplasmic characters essentially resemble 

 those o£ the 2-celled stage, but the peripheral deutoplasmic 

 network is here more strongly developed, and it is especially 

 worthy of note that it is more marked towards the lower 

 poles of the blastomeres (fig. 23), as also appears to be the 

 case in the 2-celled stage. The shell-membrane measures in 

 thickness '0072 mm. 



The next succeeding (third) cleavages are again meri- 

 dional, each of the four blastomeres becoming subdivided 

 vertically into two, not necessarily synchronously. Fig. 53. 

 PI. G, shows a side view, and fig. 54 a view from the lower 

 pole of a 6-celled e(^g, two of the blastomeres of the 4-celled 

 stage having divided before the other two. The blasto- 

 meres have moved apart, and now form an open ring 

 approximately equatorial in position, and surrounding the 

 central cleavage space, the upper opening of which is 

 occupied by the yolk-body. I have failed to obtain a 

 perfectly normal 8-celled stage, nevertheless the evidence 

 clearly shows that the iii'st three cleavage generations in 

 Dasyurns are meridional and equal, and that the resulting 

 eight equal-sized blastomeres form an equatorial ring in 

 contact with the inner surface of the sphere formed by the 

 zona and shell-membrane. 



Whilst, then, the first three cleavage generations are 

 meridional and equal, the succeeding divisions (fourth cleavage 

 genei'ation), on the contrary, are equatorial and unequal, each 

 of the eight blastomeres becoming divided into a smaller, 

 more transparent upper cell, with relatively little deutoplasm, 

 find a larger, more opaque lower cell with more abundant 

 deutoplasmic contents. In this way there is formed an 

 exceedingly characteristic 1 6-celled stage, consisting of two 



