82 THE AMPHIOXUS. 



somites in sucli a way as lias already been described 

 by Kowalevsky. By raising the object glass they 

 may be seen already defined on every side and shut 

 off; by lowering it, the connexion of their lumen 

 with the archenteroii may be distinguished (cp. also 

 Fig. 45). Especially wide is the opening through 

 which the first mesoblastic somite opens into the 

 archenteroii. 



Behind the region of the mesoblastic somites there 

 can be recognised moreover on the back view the 

 anterior and more sharply defined section of the 

 undivided mesoblast fold. At the same time the 

 latter, when being thus considered, appears towards 

 the posterior, where it is flatter, to pass direct over 

 into the wall of the archenteron. It is just the 

 lateral dorsal edges of the archenteroii, ending at 

 the posterior edge of the gastrula-mouth with the 

 two large cells, which represent the cells forming 

 the still undivided mesoblast. 



We will examine the conditions of a stage with four 

 or five mesoblastic somites by a series of transverse 

 sections. On Table VIII. Figs. 86-92, is represented 

 such a series of sections from a stage which was 

 somewhat later than that of Fig. 46. 



