10 



NOTOCHORD. 



The cells of the dorsal wall of the archenteron immediately 

 adjoining the front and sides of the blastopore have in the mean- 

 time assumed a somewhat different character from the remaining 

 cells of the archenteron, and give rise to a body which, when 

 viewed from the dorsal surface, has somewhat the form of a 

 horseshoe. This body was first observed by Metschnikoff. On the 

 elongation of the embryo and the narrowing of the blastopore the 

 cells forming this body arrange themselves as a broad linear cord, 

 two cells wide, underlying about the posterior half of the neural 

 canal (fig. 7, ch). They form the rudiment of the notochord, which, 

 as in Amphioxus, is derived from the dorsal wall of the archenteron. 

 They are seen in longitudinal section in fig. 8 II. and III. ch. 



With the formation of the notochord the body of the embryo 

 becomes divided into two distinct regions — a posterior region where 

 the notochord is present, and an anterior region into which it is not 

 prolonged. These two regions correspond with the tail and the 

 trunk of the embryo at a slightly later stage. The section of the 

 archenteric cavity in the trunk dilates and constitutes the permanent 

 . mesenteron (figs. 7, al, and 8 ill. and IV. dd). It soon becomes shut off 



from the slit-like posterior part of the 

 archenteron. The nervous system in this 

 part also dilates and forms what may be 

 called the cephalic swelling (fig. 8 iv.), 

 and the pore at its anterior extremity 

 gradually narrows and finally disappears. 

 In the region of the tail we have seen 

 that the dorsal wall of the archenteron 

 becomes converted into the notochord, 

 which immediately underlies the posterior 

 part of the medullary canal, and soon 

 becomes an elongated cord formed of a 

 single or double row of flattened cells. 

 The lateral walls of the archenteron 

 (fig. 7, me) in the tail become converted 

 into elongated cells arranged longitudi- 

 nally, which form powerful lateral muscles 

 (fig. 8 IV. m). After the formation of the 

 notochord and of the lateral muscles 

 there remains of the archenteron in the 

 tail only the ventral wall, which accord- 

 ing to Kowalevsky forms a simple cord 

 of cells (fig, 6, al). It is however not 

 always present, or else has escaped the 

 attention of other observers. It is stated by Kowalevsky to be 

 eventually transformed into blood corpuscles. The neu rente ric canal 

 leads at first into the narrow space between the above structures, 

 which is the remnant of the posterior part of the lumen of the 

 archenteron. Soon both the neurenteric canal and the caudal rem- 

 nant of the archenteron become obliterated. 



Ch-l 



Fig. 7. Optical section of 



AN EMBRYO OF PhALLUSIA MAM- 



MiLLATA. (After Kowalevsky.) 



The embryo is of the same age 

 as fig. 8 III, but is seen in longitu- 

 dinal horizontal section. 



al. alimentary tract in anterior 

 part of body; ch. notochord; me. 

 mesoblast. 



