70 THE AMPHIOXUS. 



These, starting from the front, separate into single 

 mesoblastic somites, whose cavities are parted from 

 one another. 



The cavities of the mesoblastic somites are nothing 

 else than diverticula of the archenteron. 



These discoveries of Kowalevsky which I here 

 describe are confirmed in their most important 

 respects by my own observations, and in some points 

 prosecuted with greater exactitude. 



Let us now consider again more exactly the last 

 stage of the former period of development, in which 

 the closing of the gastrula-mouth has advanced till 

 only a small remnant is left (Figs. 33, 34). The form 

 of the body is that of an egg with flattened dorsal 

 part (Fig. 33). The archenteron corresponds with 

 the outer form. The body wall consists of two layers, 

 the first being the epiblast layer, w r hich is consider- 

 ably thinner, and composed of smaller cubical clearer 

 ciliated cells. The other is the primary hypoblast 

 layer, which is considerably thicker, and is composed 

 of darker columnar cells. At the posterior end of 

 the embryo, just at the posterior edge of the gastrula- 

 mouth, lie two large hypoblast cells, which are dis- 

 tinguished by their round form from all other cells, and 



