III. 5 



HEAT 



99 



with the proper closure of the blastopore ; there is consequently 

 a large, persistent yolk-plug surrounded by a thickened blasto- 

 poric rim into which the separated halves of the medullary plate 

 and notochord are differentiated (spina bifida) (Fig. 50). The 



FIG. 50. Two transverse sections through the embyro shown in 

 Fig. 49 A. A, passes through the blastopore and yolk-plug ; B, through 

 the anterior end. d, yolk-plug ; m.p, medullary plate ; ch, notochord ; 

 mk, mesoblast. 



front end of the archenteron is, however, normally developed if 

 the temperature is not too high, and in this case the anterior 

 portion of the nervous system and 

 notochord are undivided ; pos- 

 teriorly, however, their right and 

 left halves diverge round the blasto- 

 pore, and are continued into the 

 halves of the double tail when the 

 latter is formed. Gill slits, proto- 

 vertebrae, striated muscle-fibres, 

 the pronephros and its duct, and 

 the tail fin may all be differentiated. 



The development of the organs 

 of the two sides of the body is fre- 

 quently unequal. 



At low temperatures segmentation and the closure of the 

 blastopore take place very slowly, and at cease altogether. 

 The eggs are not, however, dead, but will resume their develop- 

 ment when replaced under ordinary temperature conditions. 

 They show abnormalities, however, due to injury of the yolk ; 



H 2 



FIG. 51. Meridional section 

 of an egg of Rana fusca, de- 

 veloped at a temperature of 

 22 C. Six hours fifty minutes 

 after fertilization. M, blasto- 

 coel. 



