CLEAVAGE OF THE BIRDS EGG. 



225 



the cleavage-cells appear at the free surface of the " tread " 

 as "furrows." The two first furrows are at right angles 

 to each other, in the form of a cross (5). Two new furrows 

 then originate, which cut the former two at an angle of 45*^. 

 The tread, which is changing into the germ-disc, now forms 



Fig. 45. — Discoidal cleavage of a Bird's egg (diagrammatic, about ten 

 times enlarged). Only the formative yelk (the tread, or cicatricula, is repre- 

 sented in these 6 figures (A-F), because it alone is affected by cleavage. 

 The much larger nutritive yelk, which does not share in the cleavage, is 

 .omitted, and only indicated by the dark, outer ring. A. The first furrow 

 iseparates the parent-cell into two parts. B. These two first cleavage-cells 

 are parted by a second furrow (perpendicular to the first) into four cells. 

 C. 16 cells have originated from the 4 cleavage-cells, owing to the fact that 

 ^between the first two bisecting furrows, two other, radial furrows have 

 jappeared, and that the central portions of these 8 radial segments by 

 ,a furrow rnnning round the centre. D. A stage with 16 radial furrows and 

 about 4 concentric ring-furrows. E. A stage with 64 radial furrows and 

 iabout 6 ring-furrows. F. The whole tread has been broken up into a heap 

 jof small cells by the further formation of radial and ring furrows ; the whole 

 'now forms the lentil-shaped mulberry-germ (Morula). The separation of 

 ■the kernel always precedes the formation of the furrows. 



