UROCHORDA. I I 



floor of the groove, and, on the conversion of the groove into a 

 canal, the blastopore connects the canal with the archenteric 

 cavity, and forms a short neurenteric canal. The closure of the 

 medullary canal commences at the 

 blastopore and is thence continued 

 forwards, the anterior end of the 

 canal remaining open. The above 

 processes are represented in longitu- 

 dinal section in fig. 8 III, ;/. When 

 the neural canal is completed for its 

 whole length, it still communicates 

 by a terminal pore with the exterior. FIG. 6. TRANSVERSE OPTICAL 



T , , ,. r i i 11 SECTION OF THE TAIL OF AN EM- 



In the relation of the medullary BRYO OF PH ALLUSIA MAMMIL- 

 canal to the blastopore, as well as LATA - ( After Kowalevsky.) 



in the closure of the medullary The section is from an embryo 



c i 1 i r j .LI of the same are as ft". 8 IV. 



groove from behind forwards, the eht notoc & hord ; %.,. neural 



Solitary Ascidians agree closely with canal ; " mesoblast ; al. hypo- 



* blast of tail. 



Ampnioxus. 



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 blasto- 

 pore 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, cJi). 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. 



o 



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 III. and 

 IV. dd\ It soon becomes shut off from the slit-like posterior 



