ECHINODERMATA 



397 



shows that it is not two primitive mesenchyma cells that 

 give rise to the entire mesenchjma, but that a large num- 

 ber of cells separate from their connection with the others 

 and migrate into the blastocoele, where they subsequently 

 increase in numbers (Fig. 180 B). In Cuctimaria doliolum 

 and Holothiiria tubulo^a the formation of the mesenchyma 

 precedes gastrulation, or takes place at the same time with it 

 (Fig. 180 B). The pi-eviously somewhat thickened place of 

 the blastula indicates the region from which the migratory 

 cells detach themselves. From four to ten cells enter the 

 blastocoele, where they remain until they are forced farther 



Fig. 18').— .4, blastula staffe still within the egg-meml)r;ine, and B, larva of 

 Holothiiria tubulosa at the stage when gastrulation and the formation of the mesen- 

 chyma begin (after Selknka, from Baifodr's Comparative Embryology), ae, archen- 

 teric cavity; hi, blastula; ep, ectoderm; /, egg-membrane; hy, entoderm; mr, 

 micropyle ; ms, mesenchymatous cells ; sc, cleavage cavity. 



inwards by the first steps of invagination. The process is, 

 on the whole, the same as in Synapta. The formation of the 

 mesenchyma takes place on the same part of the larva, only 

 it occurs a little earlier. We shall see that in the sea- 

 urchins the mesenchyma takes its origin still eai-lier, even in 

 the blastula stage. 



The migratory cells, according to the observations of some authors, 

 move about in the blastocoele with great facility, so that it appears as if 

 the space between ectoderm and entoderm were filled with fluid. This 



