■I: 



402 



EMBRYOLOGY 



blastomeres becomes imperceptible even in the sixteen-cell 

 stage, and from that onwards. Ludwig, in his description of 

 the development of Asterina gibhosa, lays particular stress on 

 the fact that from the very beginning a cavity exists between 

 the cleavage spheres, and consequently the blastosphere is 

 formed at a very early period. The result of cleavage in 

 the star-fishes is also a blastula, which is formed of a layer 

 of equal-sized cells. The gastrula arises from this by invagi- 

 nation. 



The formation of the mesenchyma takes place, according 

 to the observations of Metschnikoff on Astropecten, after 



Fig. 187. 



Fig. 188. 



Figs. 187 and 188. — Blind end of the archenteronof gastrula stages of Astropecten 

 pentacanthus during the formation of the mesencliyma (after Metschnikoff). 



gastrulation is completed. The originally cylindrical cells of 

 the archenteron at its blind end become flattened out (Fig. 

 187) ; then they begin to put forth short pseudopodial pro- 

 cesses, and finally some of them detach themselves from the 

 rest. Others, usually four or five at a time, soon follow 

 these (Fig. 188). It is said that the migration of entoderm 

 cells may be so active that a more or less considerable open- 

 ing arises at the upper end of the archenteron. 



It is seen that the conditions described by Metschnikoff resemble 

 those of which we have learned through Selenka as existing in the 



