1 66 ELEMENTARY BIOLOGY. [CHAP. 



b. On the formation of the third furrow (cf. i. c). 



Median longitudinal, as at a. 

 The blastomeres, four in number as seen in 

 section; two smaller upper ones, densely pig- 

 mented ; two larger lower ones, pigmented only 

 at the periphery and yolk laden. 

 ft. The cleavage cavity (segmentation cavity); small 

 and central, in a line with the transverse furrow. 



c. At 6 — 8 hours. (Cf. i. d.) Longitudinal vertical. 

 a. The upper layer cells ; small and pigmented, 



a single layer deep, nuclei generally visible. 



ft. The lower layer cells ; large cells with little or 

 no pigment, nuclei rarely visible. 



y. The yolk granules ; aggregated in and largely 

 confined to the lower cells (vegetative pole of 

 the oosperm). 



8. The cleavage cavity ; large and irregular, inter- 

 posed between a and ft. 



d. At 30 — 38 hours. Longitudinal vertical, to pass 

 through the first trace of the blastopore. 



a. The cleavage cavity ; large and excentric, inter- 

 posed between the cells of the upper and lower 

 layers. 



ft. The lower layer cells; several rows deep, nu- 

 cleated and yolk laden; forming the main mass. 



y. The upper layer cells; now two or three rows 

 deep {epiblast or outer germinal layer) and 

 differentiated into two layers. 



Follow the course of the epiblast; it largely 

 encloses the lower layer cells and is probably 

 invaginated at the blastopore. 



