178 H. V. WILSON AND BLACKWELL MARKHAM 



this caudal continuation of the cord is markedly thicker on its 

 morphological right side than on the opposite side. On the for- 

 mer side it consists of a high lining epithelium, outside which lie 

 some rounded cells and then a little white matter, whereas on its 

 left side the wall consists only of a layer of low epithelium. The 

 tube narrows to one which still presents a relatively large lumen, 

 but the wall of which is nearly uniform in thickness, consisting 

 of a simple columnar epithelium. It extends nearly to the tip 

 of the tail. 



Summing up for the neural tube, it may be said that the neural 

 tube of head and trunk is continued directlj'- into the right tail. 

 Elsewhere symmetrical, in the right tail it gives an indication of 

 being a half-cord. In the left tail and along the left lip of the 

 blastopore there is no sign of a neural tube. 



Notochord. The cylindrical notochord of the posterior head 

 region and trunk is continued directly into the right tail with no 

 diminution in size. It extends the whole length of the tail, re- 

 maining cylindrical, but decreasing gradually in diameter in the 

 usual way. In the left tail and along the left blastophore lip 

 there is no sign of a notochord. 



Myotomes. The myotomes of the trunk are as in figure 5. 

 There are two series, but those on the right side, rm, are strongly 

 developed, those en the left side, /m, very small. The series of 

 strongly developed myotomes on the right side of the body is 

 continued into the right side of the right tail, and the series of 

 small myotomes on the left side of the body into a similar series 

 on the left side of this tail. This disproportion in size, between 

 the two series of myotomes in the tail, is as great or greater than 

 in figure 5. The series of right-hand myotomes is continued 

 almost to the tip of the tail. The series on the opposite side 

 comes to an end shortly behind the basal part of the tail. In 

 respect to the myotomes, then, the basal part of this tail is sim- 

 ilar to the trunk (fig. 5), but the greater part is more distinctly a 

 half-tail in that it lacks myotomes on one side. Doubtless these 

 were developing from before backward, when the larva was 

 killed. 



