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 (B). 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 



Pig. 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 

 separates 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 

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

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

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

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

 of 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. 



