l66 ELEMENTARY BIOLOGY. [CHAP. 



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



Median longitudinal, as at a. 



a. The hiastonicrcs, four in number as seen in 

 section ; two smaller ii])per ones, densely j^ig- 

 mented ; two larger lower ones, pigmented only 

 at the periphery and yolk laden. 



/5. The cleavage cavity {segmentation cavity); small 

 and central, in a line with the transverse furrow. 



c. At 6 — 8 hours. (Cf. i. d.) I>ongitudinal vertical. 



a. The i/J>J>ir layer cells ; small and pigmented, 

 a single layer deep, nuclei generally visible. 



/?. The loiver 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). 



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

 posed between a and /3. 



//. 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. 



y8. 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 {ipihlast 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. 



