88 THE AMPHIOXUS. 



which tlie nerve channel later on too remains still 

 for a long time open. 



In a still older embryo we see in one section of 

 tlie posterior region of the body the arching cells 

 pressed forward from both sides as far as the middle 

 line, and there coming into contact with one another 

 (Fig. 78). In the anterior region, on the other hand, 

 the middle line is still uncovered (Fig. 77). 



The arching over the medullary plate advances, 

 according to my observations, far more rapidly than 

 has been stated by Kowalevsky. I found it indeed 

 in embryos with two well formed mesoblastic somites, 

 that is, in that stage in which the embryos leave the 

 egg membranes, advanced along the whole back as 

 far as the anterior end of the first mesoblastic somite 

 (Figs. 42, 43). Through the action of reagents we 

 can easily get a separation in the line of growth, so 

 that then the wavy edges along which the arching 

 cell plates are united come plainly to view. I have 

 made a representation of this in drawing in Figs. 43 

 and 45. 



The arching plates, so far as they have touched 

 one another in the middle line and are then grown 

 into each other, are raised from the neural plate 



