ECHINODERMATA 



397 



shows that it is not two primitive mesenchyma cells that 

 give rise to the entire mesenchyma, but that a larg-e 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 Cuciimaria doliolum 

 and Holothurla tubulo<a the formation of the mesenchyma 

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

 (Fig. ISO 13). The previously somewhat thickened place of 

 the blastula indicates the region from which the migi^atory 

 cells detach themselves. From four to ten cells enter the 

 blastocoele, where they remain until they are forced farther 



Fig. 181. — .1, blastula staK ■ still within the egtr-meml)r;iiie, and B, larva of 

 Holothuvia tuhulofa at the stage when gastrulation and the formation of the mesen- 

 chyma begin (after Sklknka, from Balfodr's Coinparalive EinhnjoJogy). ae, archeii- 

 teric cavity; hJ, blastula; ep, ectoilerm ; /, egg-membi'ane; hy, entoderm; inr, 

 micropyle; ras, mesenchymatous cells ; (•o, cleavage cavity. 



inwai'ds 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 earlier, even in 

 the blastula stage. 



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

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

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



