i io THE AMPHIOXUS. 



ment of the notocliord is to a slight extent later than 

 that of the second mesoblastic somite. This may 

 indeed be seen on a comparison of Figs. 87 and 88, 

 where the folding of the median hypoblast plate in 

 the neighbourhood of the first mesoblastic somite is 

 much less expressed than in the region of the suc- 

 ceeding section. 



Further, we see in the stage with six mesoblastic 

 somites in the region of the first of these the 

 notochord slit still well marked (Fig. 98), while it 

 has already vanished in the region of the second and 

 following mesoblastic somites, through the ingrowth 

 of the cells (Fig. 100). 



The delay in the notochord formation in the region 

 of the first mesoblastic somite is, however, quite un- 

 important, and only to be seen in a few stages on good 

 transverse sections. 



Far more remarkable is the delay of the develop- 

 ment of the notochord in the most anterior end of 

 the body. "We see, reverting to earlier stages, in Fig. 

 86, where a section is represented which has to do 

 with the anterior opening of the neural canal, no trace 

 here, as yet, of that channel which introduces the 

 formation of the notochord. 



