ECHINODERMATA 



399 



vegetative pole (Fig. 181). The cleavage cavity is diminished 

 in size as a i-esult of the considerable elongation of the cells 

 at the vegetative pole, and the embryo becomes slightly oval 

 in shape. A long flagellum makes its appearance on each of 

 the cells of the blastoderm ; the embryo begins to rotate within 

 the vitelline membrane, and finally breaks through the latter 

 to swarm oat as a larva. At the same time a more active 

 multiplication of the cells begins at the thickened part of 

 the blastula, as the result of which some of them soon be- 

 come foi^ced into the cleavage cavity (Fig. 182), where, like 

 Amoebee, they creep about as migratory cells. These are 

 followed by othei's ; after multiplying rapidly in the blasto- 

 ccele, they almost fill it. 



Fi&. 183. 



Fio. 18i. 



Figs. 183 and 184. — Blastula stages showing the commencement of formation of 

 mesenchyma (primitive mesenchyma cells) (after Selenka and Hatschbk). The 

 flagella are omitted here, as also in Fig. 185. 



We have described the formation of the mesenchyma as it appeared to 

 us from personal observations (No. 26). However, other statements have 

 been made on this point. According to the observations of Selenka and 

 Hatschek (No. 54), there arises some time after the swarming out of the 

 larva a funnel-like depression (Figs. 183 and 184) at the vegetative pole, 

 which owes its i^roduction to the shortening and thickening of two cells 

 lying at that pole of the blastula. These two cells were looked upon by 

 the observers last named as the primitive cells of the mesenchyma ; to this 

 interpretation Fleischmann also adheres. According to him, there are 

 present at the vegetative pole four such primitive mesenchyma cells, 

 which have been differentiated there during cleavage. The primitive 

 mesenchyma cells are said to correspond to the primitive cells of the 



