W. K. BROOKS ON THE GENUS SALPA. 289 



although at the stages shown in Plates XIII and XIV the follicular rudi- 

 ment of the notochord, 19, and that of the nervous system, 30, are still 

 distinct from each other. At the stage which is shown in Plate XVI and 

 Plate XVII, however, this region of the body is occupied by a single mass 

 of vacuolated and degenerating cells, the eleoblast, k, the later history of 

 which has already been described on page 38. It seems to represent both 

 the chorda rudiment and the nerve rudiment of the younger stages, and 

 I have already shown, pp. 38 and 128, that it is homologous with the tail 

 of ascidians and doliolum. 



The blastomeres which extend downwards at the anterior end of the 

 embryo, 3, in Plate XII, Figs. 1, 2, 4, 5 and 6, form the rudiment of the 

 ganglion which is represented in all the plates by s. It is enclosed in a 

 mass of follicle cells, which are at this stage continuous above with those 

 of the somatic layer of the follicle, while below they are continuous 

 with the follicle cells of the visceral mass. It is probable that its con- 

 nection with the somatic layer is primary, and with the visceral layer 

 secondary, for in older embryo of Salpa pinnata it loses its connection 

 with the somatic layer, while it at the same time comes into more 

 intimate relation to the visceral layer, as Plates XVI, XVII and XVIII 

 show. 



In an embryo of Salpa hexagona, at about the stage shown in 

 Plate XII, it is distinct from the visceral mass, and so intimately united 

 to the somatic layer that, as Plate XI, Figs. 4, 5 and 6, s, show, its folli- 

 cular portions seem to be an invagination from the somatic layer, 7. 



In ordinary tunicates the ganglion consists of two structures in 

 intimate relation to each other the ganglion proper, which is formed by 

 the invagination of the dorsal ectoderm, and the so-called sub-neural 

 gland, which is a diverticulum from the pharynx. 



The connection between the follicle cells of the ganglionic rudiment 

 and the somatic layer of the follicle is easily intelligible as a record of 

 the primitive origin of the ganglion from the ectoderm. 



After the ganglionic rudiment has lost this connection and has 

 united with the visceral layer, the cavity of the pharynx becomes pro- 

 duced into it, as is shown at s in Plate XVIII, Figs. 2, 3 and 8. In Fig. 1 

 of Plate XVII the pharynx consists of two parts, a very broad portion 

 which lies under the perithoracic tubes, and a much narrower part which 

 runs forwards and meets the ectoderm at the point where the mouth is 

 afterwards formed. The cavity of the ganglionic rudiment opens into 

 the roof of this narrow oral division of the pharynx. 



