404 EMBRYOLOGY 



place from the archenteron, bat only after gastrulation 

 (Barrois, No. 6 ; Bury, No. 7), as in Sjnapta and the Aste- 

 roidea. The cells of the archenteron, especially those lying 

 at its apex, lose their regular arrangement, apparently as the 

 result of a rapid cell-proliferation occurring at this point, so 

 that the arch ente ion no longer presents strictly a single 

 layer of cells, but is composed of cells irregularly grouped. 

 A large number of these cells migrate into the cleavage 

 cavity, and form the mesenchyma (Fig. 189). 



The various kinds of mesenchyma formation are not so different from 

 one another as they at first sight appear to be. It is always substantially 

 the same region of the blastula from which the mesenchyma talces its 

 origin, the only difference being that in the one case this has already 

 undergone invagination, while in the other the invagination does not 

 take place until later. It seems as if those methods of mesenchyma 



formation were the more primitive in 



which the cells take their origin from the 



archenteron. Later the coelomic sacs are 



also developed from the archenteron, and 



thus the development of the mesenchyma 



and the mesoderm could be correlated. 



There are found various transitional stages 



of this anticipatory misplacement, i.e., 



between the origin of the mesenchyma 



from the archenteron and its development 



Fig. 189.— Development of the from the thickened pole of the blastula, 



mesenchyma in the gastrula of as, for example, in the Holothurians. In 



Antedon rosacea (after Bury). gynapta the mesenchyma arises from the 



' ■ archenteron, whereas in Holothuria it 



begins at the same time as gastrulation. 



The metamorphoses vi^hich the archenteron of Antedon 

 undergoes are different from those in other Echinodermata 

 in so far as the blastopore does not become the anus, but 

 closes, after which the archenteron is constricted off from 

 the ectoderm and lies as a spacious sac in the interior of the 

 embryo. Mouth and anus arise as new formations, but not 

 until much later. There is certainly a connection between 

 these complicated formative processes and the fact that the 

 larva of Antedon abandons the egg-membrane much later 

 (the seventh day of development) than other Echinoderm 

 larvae. 



