12 



NOTOCHORD. 



ch- 



part of the archenteron. The nervous a i 



system in this part also dilates and 

 forms what may be called the ce- 

 phalic swelling (fig. 8 IV.), and the 

 pore at its anterior extremity 

 gradually narrows and finally dis- 

 appears. In the region of the tail 

 we have seen that the dorsal wall of 

 the archenteron becomes converted 

 into the notochord, which imme- 

 diately 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 archen- 

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

 converted into elongated cells ar- 

 ranged longitudinally, which form 

 powerful lateral muscles (fig. 8 IV. 

 tri). After the formation of the noto- 

 chord and of the lateral muscles 



there remains of the archenteron in the tail only the ventral wall, 

 which according to Kowalevsky forms a simple cord of cells 

 (fig. 6, at). 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 neuren- 

 teric 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 remnant of the archenteron become obliterated. 



During the above changes the tail becomes considerably 

 elongated and, owing to the larva being still in the egg-shell, is 

 bent over to the ventral side of the trunk. 



The larva at this stage is represented in a side view in fig. 8 

 IV. The epidermis is formed throughout of a single layer of 

 cells. In the trunk the mesenteron is shewn at dd and the 

 dilated part of the nervous system, no longer communicating 

 with the exterior, at n. In the tail the notochord is shewn at 

 ch, the muscles at m, and the solid remnant of the ventral wall 



FIG. 7. OPTICAL SECTION OF 

 AN EMBRYO OF PHALLUSIA MAM- 

 MI LLAT A. (After Kowalevsky.) 



The embryo is of the same age 

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

 dinal horizontal section. 



al. alimentary tract in anterior 

 part of body ; ch. notochord ; me. 

 mesoblast. 



