1 66 ELEMENTARY BIOLOGY. [CHAP. 



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



Median longitudinal, as at a. 

 a. The d/astomereSy 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. 



/S. 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. 



^. 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 fi. 



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. 



/?. The lower layer cells; several rows deep, nu-j 



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. 



