ON THE EMBRYOLOGY OF THE ECTOPEOCTA. 9 



statements of previous observers, to which we must now devote 

 our attention. 



It is practically certain .that the separation of a mass of 

 endoderm-cells from the alimentary canal does not take place, 

 in Alcyonidium, in the region of the pyriform organ, as described 

 by Repiachoff (15) in Tendra. The oesophagus is sharply dis- 

 tinguished, except in the earlier stages, from the tissues im- 

 mediately in front of it by the large number of yolk-spheres 

 present in its walls {vide figs. 2 and 3), and it is hardly to be 

 supposed that the fibrillar tissue in front of the oesophagus 

 could in any case be derived from the latter. I feel myself 

 obliged to doubt the accuracy of Repiachoff's statements on 

 this part of the development of Tendra, although it will be 

 noticed that in other respects the description I have given 

 agrees very closely with that of Repiachoff. 



Vigelius (16) has published certain figures of sections of 

 ^-it^'M^a-erabryos which suggest the possibility of the occurrence, 

 in that genus, of a nervous system similar to that oi Alcyonidium. 

 In fig. 49 of PI. XXVII Vigelius shows a layer of fibrils running 

 round the pyriform organ in exactly the same position as the 

 nerve nv. in my own fig. 4. Vigelius does not, however, in his 

 description call any attention to the existence of these supposed 

 nerve-fibrils. Again in fig. 14 of PI. XXVI, Vigelius shows that 

 the structure of the "calotte" of the larva is almost identical with 

 that represented in the dorsal region of fig. 4 of the present 

 paper; — i.e. that an internal proliferation of epiblast cells on 

 each side of the middle line apparently takes place in this 

 part of the larva. These cells may perhaps correspond with 

 the structure identified as the brain in Alcyonidium, and the 

 similarity between this genus and Bugula is rendered still more 

 striking by the existence, in the latter (as is shown by the 

 figure of Vigelius), of a central region which does not take 

 part in the proliferation, and which projects as a wedge-like 

 mass into the supposed nervous tissue (c/. my own fig. 4). 

 Vio-elius does not, however, show that there is any connection 

 between the cells proliferated off by the "calotte" and the fibres 

 round the p) liform organ. 



