398 



EMBRYOLOGY 



view is advocated, for example, by Ludwig (No. 35), whereas other inves- 

 tigators (Hensen, Selenka) ascribe a gelatinous consistency to the con- 

 tents of the cleavage cavity. 



Echinoidea. — According to the recent investigations of 

 Selenka on Strongylocentrotus lividvs, SphferecMnus granu- 

 laris, and Echinus microtuberculatus, and of Fleischmann on 

 Echinocardium cor datum, cleavage does not take place so 

 regularly in the sea-urchins as in the Holothurians. During 

 the first four cleavage phases only do the farrows extend to 

 all the blastonieres ; then for a time some of the elements of 

 the circles of cells now present take no part in the further 

 cleavage, so that they soon greatly surpass in size the blasto- 



FiG. 181. 



Fig. 181. — Blastula stage of 

 StrongyJocentrotus lividus (after 

 Selenka). The ciliation of the 

 larva is omitted in this and most 

 of the following figures. 



Fig. 182. 



Fig. 182. — Blastula stage of StrongyJo- 

 centrotus lividns showing the migration 

 of mesenchyma cells (after Korschklt). 

 The flagella are represented too stout. 



meres at the opposite pole of the egg. Since, however, the 

 difference in the size of the blastomeres disappears as cleav- 

 age progi^esses, a regular blastula, consisting of a layer of 

 rather tall cells of nearly equal size, results fi'ora cleavage 

 even in the Echinoidea (Fig. 181). 



In the Echinoidea the formation of the mesenchyma regu- 

 larly precedes gastrulation. At the end of cleavage there 

 occur in the blastula a flattening of the cells at the ajiimal 

 pole and, on the other hand, a thickening of those at the 



