W. K. BROOKS ON THE GENUS SALPA. 33 



epithelial lining of the pharynx is gradually completed and extended 

 until at last it becomes continuous, as shown in Fig. 9, in which 6 is the 

 cavity of the pharynx and 15 the body cavity. Even after this epithelium 

 is well denned, it is here and there interrupted by a follicle cell, with its 

 two transparent nuclei, as is shown at 8 in Fig. 10. This figure is part 

 of Fig. 2 of Plate XVII. 



Other blastomeres migrate upwards along the gill-slits, under the 

 somatic cells, as shown in Fig. 6 of Plate XLII, which is from an embryo 

 at the stage of Plate XXII. These cells multiply, and finally build up a 

 continuous epithelium in the gill-slits, as shown in Plate XLII, Fig. 9, 

 which is part of the section shown in Plate XVI, Fig. 3. In Fig. 9, gr' v is 

 the cavity of the gill-slit, a; is a group of degenerating somatic follicle 

 cells, 6' is the blastodermic epithelium, 75 is the body cavity, 8 the visceral 

 follicle cells, A mesoderm cells, a the ectoderm, and &' the cells of the 

 degenerating epithelial capsule. The oesophagus, stomach and intestine 

 are formed from a diverticulum from the posterior wall of the pharynx, 

 a little to the right of the middle line, as shown at a very early stage at q 

 in Plate XVII, Fig. 2, and more magnified in Plate XLII, Fig. 10, q. The 

 opening of this diverticulum becomes the oesophagus, shown at q in Plate 

 XVIII, Figs. 6, q and 8, q, and it elongates to form the stomach and 

 intestine, but as it soon becomes twisted it cannot be described in detail 

 without a greater number of figures than the subject seems to merit, as 

 the only point which seems noteworthy is that the digestive tract is a 

 secondary outgrowth from the pharynx. As the blastodermic epithelium 

 of the gill is formed, the intestine extends forwards into it, as is shown in 

 Plate XLI, Figs. 3 and 5, and at p in Plate XXXV. The anus is not 

 formed until the embryo is well advanced, when it breaks through into 

 the cloaca at the point marked p" in Plate XXXV, on the middle line at 

 the anterior end of the cloaca. Plate XIX, Fig. 1, is a horizontal section, 

 reversed in the drawing, through the gill of an embryo a little older than 

 Plate XXXV, showing the intestine p in the gill o, and the opening of 

 the oesophagus q, on the right side at the base of the gill. 



At a very early stage, Plate XVII, Figs. 1 and 2, a stomodaeal invo- 

 lution of the ectoderm z grows inwards to meet a corresponding out- 

 growth from the pharynx, although the aperture of the mouth is not 

 formed until the embryo is well advanced. In Plate XXXV the stomo- 

 daeum z is shown with its inner end covered up by the unbroken epithe- 

 lium of the pharynx. Plate XLV, Fig. 8, is a vertical section through 

 the mouth at the time of its appearance, from an embryo like Plate XLI, 

 Fig. 5. 



