3 86 



AN INTRODUCTION TO ZOOLOGY 



each being hemispherical and do not separate as much as in Amphi- 

 oxus. The second cleavage, also holoblastic, takes place in a plane 

 passing through the centres of the animal and vegetal poles in a 

 plane at right angles to the first and results in the formation of 

 four blast omeres. The third division, as in Amphioxus, is in a plane 

 at right angles to the other two, but although complete, is well above 

 the equator and cuts off four smaller cells at the animal pole from 



ttc. 



yli.c. 



FIG. 131. Rana, early stages segmentation. From Dendy. 



/., the fertilized ovum ; II.-V., segmentation of the ovum ; VI., blastula ; VII., modified 

 blastula, with wall composed of more than one layer of cells ; VIII., commencement of gastru- 

 lation ; IX., the modified gastrula stage (VI. -IX. in vertical section), blc., blastoccel or segmenta- 

 tion cavity ; bp., blastopore ; ent., enteron ; ep., epiblast ; hyp., hypoblast ; u.l.bp., upper lip 

 of blastopore ; yk.c., yolk-containing cells. 



four much larger ones at the vegetative pole. The fourth cleavage 

 is initiated by the appearance of a pair of furrows bisecting the 

 apical angles of the cells of the animal pole and it extends more 

 slowly down over the four vegetal cells, but finally produces sixteen 

 cells. Somewhere about this stage the segmentation proceeds less 

 regularly and takes place more rapidly at the animal pole, as if it 

 were hindered at the vegetative pole by the presence of too much 



