532 MAYNARD M. METCALF, 



of the rapheal nerve cord are derived directly from the cortical cells 

 of the brain ; that in others they are derived from the rapheal duct ; 

 while in still other species they are derived from a common mass of 

 cells formed by the fusion of gland and brain. Of course in those 

 species which have no rapheal duct the cells of the rapheal nerve 

 cord can have no connection with the rapheal duct. Nor in these 

 cases do we usually find the rapheal nerve cord arising from the 

 area of fusion between ganglion and gland, if such be present. (It is 

 by no means present in the adults of all species.) 



A first sight such diversity seems very puzzling, yet the ex- 

 planation is, I believe, very simple. The nerve tissue and that of the 

 neural gland are not wholly independent in the Tunica ta. It is 

 well known that the Ascidian ganglion and neural gland arise from 

 a common source, from that portion of the nervous system of the 

 tadpole which lies just behind the sensory vesicle. The gland arises on 

 one side of the nerve tube by the great proliferation of the cells of 

 its wall, while the cells of the opposite wall proliferate in the same 

 way to form the neural gland. The fusion between the brain and the 

 neural gland, found in the adults of so many species, is, I believe, 

 a reminiscence of their origin from a common source. In the same 

 way we may account for the intimacy of relation between the rapheal 

 duct and rapheal nerve cord, pointed out above. 



The ganglion cells of the rapheal nerve have had a very com- 

 plicated history in a form like Phallusia in which they are derived 

 from the rapheal duct. Certain cells of the larval nerve tube were 

 pushed out to form the neural gland, a portion of these cells extended 

 backward ^) until they came in contact with the fibres of the rapheal 

 nerve. Here they loose their regular arrangement and become the 

 ganglion cells of the nerve. There is no evidence that these parti- 

 cular cells, even though a part of the gland, were ever functional as 

 glandular cells. The corresponding cells, however, in many other 

 species are functional gland cells (compare Cynthia papulosa). 



In this connection an interesting query presents itself. The 

 dorsal raphe corresponds to that region along the back of the tad- 

 pole in which the elongated tubular central nervous system lies (cf. 

 Fig. 65, Plate 38). The cloaca arises later at the sides of and dorsal 

 to this region, but the original tissue along the course of the nerve 



1) I assume here that rapheal duct arises in the ontogeny as an 

 outgrowth of the gland. This is probable, but not proven. 



