W. Z. BROOKS ON THE GENUS SALPA. 269 



invaginations of the somatic layer of the follicle. 4. The dorsal hemi- 

 sphere of the somatic layer of the follicle is closely wrapped by the 

 epithelial capsule, cut B, except on the dorsal middle line where the 

 ectodermal blastomeres lie between, and an approximately equal area of 

 the somatic layer on the ventral hemisphere forms the inner boundary 

 or roof, 10, of the cavity of the placenta, y', which separates this portion 

 of the follicle from the supporting ring, 23. 5. "A so-called blood bud " 

 has been formed in Salpa pinnata at 24 in the cut, and in Fig. 9, as an 

 outgrowth from the center of the ventral hemisphere of the somatic 

 follicle. This brief description of the embryo of Salpa pinnata at this 

 stage of development gives most of the facts which we need as a prepa- 

 ration for studying the history and fate of the follicle, but there are a 

 few other embryological features which must be noted. 



The two perithoracic tubes soon lose their external openings and 

 their connection with the somatic layer of the follicle, and, as already 

 pointed out, they move inwards until they come to lie side by side on 

 the middle line above the atrium, as is shown in cut C and also in 

 Plate XIII, Fig. 6, and Plate XXII, Fig. 4. Finally they degenerate, and 

 the atrial aperture is formed as a new opening on the dorsal middle line, 

 as shown in Plate XVII, Figs. 6 and 7. The cavity of the pharynx is 

 hollowed out by the disintegration of the follicle cells of the visceral 

 layer, as is shown at Plate XLII, Fig. 8, and the gill-slit or communi- 

 cation between the cavity of the pharynx and the cavity of the peri- 

 thoracic tube is formed in the same way, as is shown at x in Plate 

 XLII, Fig. 6, by the disintegration of the intervening layer of the 

 follicle cells. The ectoderm, Plate XLII, Fig. 11, a, is formed by the 

 growth and multiplication of the ectodermal blastomeres. It gradually 

 stretches outwards and downwards, as shown at a in Plate XVIII, Figs. 

 4, 5 and 6, but it never covers up the ventral hemisphere of the somatic 

 follicular layer. Even at birth the ectoderm is absent in the region of 

 the placenta, and it is not until all traces of the follicle have disappeared 

 after birth that the ectoderm covers up the spot which was occupied by 

 the opening of the placenta. 



SECTION 3. TJie Fate of the Follicle. 



The follicle may be divided, for purposes of description, into five 

 regions, each of which has its own peculiar relations to other structures 

 and its own peculiar history. 



