8 J. P. HILL. 



enormous thickness of the albumen layer], in which the four 

 equal-sized blastomeres are radially arranged round a cleavage 

 cavity and are conical in form, their upper ends being more 

 pointed, their lower ends thicker and richer in yolk-material. 

 The nucleus of each is excentric, being situated nearer the 

 upper pole. [This description is applicable word for word to 

 the4-celled stage of Dasyurus.] 



An 8-celled stage (tig. 6) is next described, seven of the 

 blastomeres being equal in size and one being smaller. They 

 are arranged somewhat irregularly in two circles. [This stage 

 I regard as abnormal both in respect of the arrangement of 

 the blastomeres and the occurrence of irregularity amongst 

 them.] Selenka (p. 119) thought it probable that the third 

 cleavage planes cut the first two at right angles and divided 

 each of the first four blastomeres into a smaller ectodermal 

 cell and a larger more granular entodermal, but states that 

 he was unable to establish this owing to the opacity of the 

 albumen-layer. [My observations show that it is the fourth 

 cleavage in Dasyurus, not the third, which is equatorial, un- 

 equal, and qualitative, and that even then the cells formed are 

 not ectodermal and entodermal insignificance. The albumen 

 is normally never opaque.] 



A 20-celled stage is mentioned, but not described, since it 

 suffered in preparation. It is said to have a large entoderm 

 cell in the cleavage cavity. [A statement of very doubtful 

 value, since the blastomeres were admittedly pressed together 

 and probably displaced by the shrunken egg-membranes.] 



The next stage described is a spherical "gastrula" (Taf. xvii, 

 figs. 7,8), composed of forty-two cells with an open "blasto- 

 pore" at the vegetative pole, a smaller opening at the animal 

 pole, and a large " ur-eutoderm " cell in the cleavage-cavity, 

 just inside the "blastopore." The wall of the "gastrula" 

 consists of cells gradiuited iu size ; those in the region of the 

 blastopore are the largest and richest in deutoplasm, those at 

 the opposite pole are the smallest and most transparent. [This 

 is a very characteristic stage in the formation of the blastocyst, 

 with which I am quite familar in Dasyurus. Selenka's speci- 



