UROCHORDA. 



The segmentation is followed by an invagination of nearly the 

 same character as in Amphioxus. The blastosphere resulting from 

 the segmentation first becomes flattened on one side, and the cells 

 on the flatter side become more columnar (fig. 8 i.). Very shortly 

 a cup-shaped form is assumed, the concavity of which is lined by 

 the more columnar cells. The mouth of the cup or blastopore next 

 becomes narrowed; while at the same time the embryo becomes 

 oval. The blastopore is situated not quite at a pole of the oval but 

 in a position which subsequent development shews to he on the 

 dorsal side close to the posterior end 

 of the embryo. The long axis of the 

 oval corresponds with the long axis 

 of the embryo. At this stage the 

 embryo consists of two layers ; a 

 columnar hypoblast lining the central 

 cavity or archenteron, and a thinner 

 epiblastic layer. The dorsal side of 

 the embryo next becomes flattened 

 (fig. 8 II.), and the epiblast covering 

 it is shortly afterwards marked by 

 an axial groove continued forwards 

 from the blastopore to near the front 

 end of the body (fig. 5, mg). This is 

 the medullary groove, and it soon 

 becomes converted into a closed canal 

 — the medullary or neural canal — 

 below the external skin (fig. 6, n.c). 

 The closure is effected by the folds on each side of the furrow 

 meeting and coalescing dorsally. The original medullary folds fall 

 into one another behind the blastopore, so that the blastopore is 

 situated on the 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 blas- 

 topore and is thence continued forwards, 

 the anterior end of the canal remaining 

 open. The above processes are repre- 

 sented in longitudinal section in fig. 

 8 III, n. When the neural canal is 

 completed for its whole length, it still 

 communicates by a terminal pore with 

 the exterior. In the relation of the 

 medullary canal to the blastopore, as 

 well as in the closure of the medullar}' 

 groove from behind forwards, the Soli- 

 tary Ascidians agree closely with Am- 

 phioxus. 



Fig. 5. Tkansverse section 

 through the front end of an em- 

 BRYO OF PhALLUSIA MA5IMILLATA. 



(After Kowalevsky.) 



The embryo is slightly younger 

 than that represented in fig. 8 iii. 



mg. medullary groove; al. ali- 

 mentary tract. 



n.r 



me- 



FiG. 6. Transverse optical 



SECTION OP THE TAIL OF AN EM- 

 BRYO OF Phallusia MAMMILLATA. 

 (After Kowalevsky.) 



The section is from an embrj'o 

 of the same age as fig. 8 iv. 



ch. notochord ; n.c. neural 

 canal; me. mesoblast ; al. hypo- 

 blast of tail. 



