106 



EXTERNAL FACTORS 



perpendicular, instead of parallel, to the other three, or two 

 perpendicular to two, or all irregular; in the next phase the 

 formation of micromeres was partially or wholly suppressed. 

 Nevertheless these abnormally segmented eggs produced perfectly 

 normal Plutei. 



It is also possible for nuclear division to continue while cell- 

 division is suppressed, as a result of exposure to high tempera- 

 tures (Driesch) (Fig. 57). 



FIG. 58. The effect of heat upon the development of Sphaerechinus 

 grannlaris. a, exogastrula ; fc, exogastrula, with tripartite gut ; c, Pluteus, 

 with tripartite gut ; (d) Anenterion, with stomodaeum, but no gut. 

 (After Driesch, 1895.) 



By exposing the blastulae to the same high temperature 

 Driesch brought about a very interesting malformation, an Anon- 

 terion (Fig. 58). The archenteron was formed and constricted 

 into the normal three portions, but it was evaginated instead of 

 invaginated. Later on it shrank up and disappeared ; the rest 

 of the embryo, however, became a Pluteus, with a stomodaeum. 



Vernon finds the optimum temperature for Echinoid larvae 



