58o 



AMPHIBIA. 



arch. 



tured, and the ovum be kept still, the other half will, 

 according to Roux, form a one-sided half-embryo. At 

 a certain stage Roux's half-embryo regenerated the missing 

 half, usually by re-vitalising the remains of the cell which 

 was punctured. If the ovum be shaken about after punctur- 

 ing, a readjustment of material is effected, and a half-sized 

 embryo is formed (Morgan). The second cleavage is also 

 vertical, and at right angles to the first, dividing an anterior 

 from a posterior half. The third cleavage is equatorial, at 

 right angles to the first two, dividing the dorsal region from 

 the ventral. 



The segmentation is total but unequal, and results in the 



formation of a ball of 

 cells, those of the upper 

 hemisphere being smaller 

 and more numerous than 

 the yolk - laden cells 

 below. Within there is 

 a small segmentation 

 cavity. Since the 

 presence of yolk acts as 

 a check on the activity 

 of the protoplasm, we 

 can understand why the 

 smaller cells continue to 

 divide much more 

 rapidly than the large 

 yolk - containing cells, 



and so how the smaller epiblastic cells gradually spread 

 over the egg, covering in the larger ones. At one 

 point, where upper and lower cells meet, a groove is 

 formed. According to the older view, at this point the 

 small cells are invaginated, and so form a cavity ; according 

 to recent research, the cavity is simply formed by the split- 

 ting of the large cells. However this may be, the cavity, 

 which is the archenteron or embryonic gut, rapidly enlarges 

 at the expense of the segmentation cavity, which soon dis- 

 appears. The groove becomes a circular aperture in the 

 epiblast, which has now spread over the whole egg except 

 at this spot, the blastopore. The embryo elongates slightly, 

 but the mass of yolk-laden cells which lie on the floor of the 



s. c. 



FIG. 284. Gastrula stage of newt. After 

 Hertwig. 



ep., Epiblast ; hyp., hypoblast ; arch., archen- 

 teron ; y. t yolk-cells; s.c., segmentation 

 cavity. 



