334 ARTHUR E. SHIPLEY. 



cells_, as has been suggested in the case of the Teleostei. It 

 appears at first anteriorly and extends backward, and for some 

 little time the walls of the lumen are by no means sharply 

 defined. Processes from the cells lining the canal ])roject into 

 its cavity and suggest the idea that they have been torn out 

 from between the cells of the other side. 



The neural cord remains solid at its posterior end for some 

 time, and here it becomes fused with the surrounding struc- 

 tures in a somewhat remarkable way. It does not fuse round 

 the blastopore as Scott describes, indeed it is not easy, con- 

 sidering its mode of origin, to see how it could ; and there is 

 no hollow neurenteric canal. Figs. 14 and 15 represent two 

 sections taken through a larva just after hatching. Fig. 14 is 

 through the region of the blastopore. It shows the neural 

 cord with its canal already formed ; beneath this lies the uoto- 

 chord, and beneath this again a solid rod of cells which is con- 

 tinuous with the subuotochordal rod and the dorsal hypoblast. 

 This latter structure is the solid postanal gut. The mesoblastic 

 plates are seen separating ofi" from the hypoblast yolk-cells 

 which occupy the remaining space with the epidermis. Dor- 

 sally this is produced to form the dorsal fin. Fig. 15 repre- 

 sents a section through the tail a little posterior to the blasto- 

 pore. Here the neural cord, notochord, and postanal gut have 

 fused into a rod-like mass of tissue which is ventrally con- 

 tinuous with the hypoblast cells ; a few sections posterior to 

 this none of the three embryonic layers are distinguishable 

 except the epidermal portion of the epiblast. A longitudinal 

 median section through the tail is represented in fig. 20. This 

 shows the mass of iudiff'erent tissue which lies in the tail and 

 which by internal differentiation gives rise, as the tail grows, 

 to mesoblastic somites, neural cord and postanal gut. This 

 mass of tissue, which in many respects reminds one of the 

 growing point in a plant, may be called the primitive streak. 

 It is perhaps worth while to point out that it lies at what was 

 originally the anterior lip of the blastopore. 



A similar mass of tissue formed by the fusion of the pri- 

 mary layers has been described by Balfour and Parker in 



