W. K. BROOKS ON THE GENUS SALPA. 41 



where the visceral follicle cells are continuous with those of the somatic 

 layer. In Fig. 2, s, they are shut into the visceral layer, and the gang- 

 lionic rudiment lies in the body cavity, 15. At a still lower level, Figs. 6 

 and 7, the blastomeres pass without any line of demarcation into those 

 of the pharynx. 



In an embryo a little older, Plate XIII, we find essentially the same 

 condition. In Fig. 3, s, the ganglion cells are extra-follicular; in Figs. 4 

 and 5, s, they gradually pass into the visceral layer of the follicle; in 

 Figs. 6, 7 and 8, the ganglionic rudiment is within the body cavity, and 

 in Fig. 9 it becomes continuous with the rudiment of the pharynx. In 

 Plate XIV, Fig. 4, s, it begins to be shut in dorsally and separated from 

 the somatic layer, while a cavity appears within it, as shown in Figs. 5 

 and 6, and in Plate XXII, Fig. 2, s. The cavity of the pharynx, c, is 

 formed at the same time, and at first consists of a broad chamber, Plate 

 XIV, Figs. 8 and 9, and a much narrower anterior portion, which is 

 shown in Fig. 8 and, at an older stage, in Plate XVII, Fig. 1. This 

 narrow portion, which is the stomodaeal portion of the pharynx, soon 

 loses its distinctness, as the whole pharynx widens out directly up to the 

 mouth, as shown in Plate XIX, Figs. 4 and 5, but before this takes place 

 the stomodaeal diverticulum lies directly under the ganglion, as will be 

 seen by comparing Figs. 6, 7 and 8 in Plate XIV, although the cavities 

 of the two structures are at first separate, as shown in the intermediate 

 section, Fig. 7. 



The follicle cells which separate them soon disappear, and the cavity 

 of the ganglion opens into the cavity of the stomodaeal diverticulum of 

 the branchial sac. This is the case at the stage which is shown in Plates 

 XVI and XVII, where the cavity of the ganglion is shown at s in Figs. 2 

 and 3 of Plate XVI, while the stomodaeal diverticulum is shown in the 

 same place in Fig. 1 of Plate XVII. The study of the intermediate 

 sections, which are not figured, shows that there is now no partition 

 between the two, and the relations between the two structures are as 

 shown in a longitudinal section of an embryo a little older, in Plate 

 XVIII, Fig. 8, s and z. The opening of the ganglion into the pharynx is 

 also shown in Plate XVIII, Fig. 2, where s is the cavity of the ganglion 

 and c the cavity of the pharynx. Plate XXXV, s, shows the ganglion at 

 a still older stage, and its opening into the pharynx at t, in longitudinal 

 section. 



The pericardial rudiment first makes its appearance in the longitu- 

 dinal plate of the visceral mass, at/, in Plate XIV, Figs. 8 and 9, between 



