22O THE ASCIDIANS. 



Body-cavity of an Ascidian and C&lom of Amphioxus. 



We must now endeavour to show how the body-cavity 

 of the Ascidian can be brought into genetic relationship 

 with the coelom of Amphioxus. The question of the 

 absence of metamerism in connexion with the origin of 

 mesoblast in the Ascidians need not detain us, since it is 

 so obviously correlated with their mode of life. It may 

 safely be asserted that the Ascidian mesoderm, as a whole,, 

 is homologous with that of Amphioxus as a whole, but in 

 the details of its origin and fate it is widely different. 



If we figure to ourselves the coelomic epithelium of 

 Amphioxus losing its character as a membrane and break- 

 ing up into its constituent cells, which would then lie loosely 

 in the body-cavity, we should have essentially the same 

 condition of things as in the Ascidians. There are numer- 

 ous precedents in the animal kingdom for such a disinte- 

 gration of an epithelial membrane. 



A most perfect instance of it has been described by DR. 

 R. VON ERLANGER* in connexion with the origin of the 

 mesoderm in the fresh-water snail, Paludina vivipara. Here 

 the mesoderm appears at first in the form of a median 

 bilobed archenteric pouch of relatively large dimensions. 

 Soon, however, the cells forming the wall of the pouch begin 

 to assume irregular shapes, and so disturb the contour of 

 the epithelium, and eventually they break apart entirely 

 and fill every nook and corner of the available space with 

 a loose mesenckyme. Similar out-wanderings of cells from 

 an epithelial wall, though not often of such a complete 

 character as the instance above cited, are by no means 

 infrequent. 



* Zur Entwicklung der Paludina vivipara. Parts I. and II. Morpholo- 

 gisches Jahrbuch, XVII. 1891. 



