W. K. BROOKS ON THE GENUS SALPA. 267 



through the two folds of the embryo sac, 21 and 22, and the epithelial 

 capsule, &', as before, and then on the right side it passes through the 

 external opening of the right perithoracic tube. This opening does not 

 communicate with the cavity of the embryo sac, however, since it is 

 covered by the epithelial capsule. The section then passes through the 

 follicular roof of the atrium, and after crossing the middle line it cuts 

 the left perithoracic tube below the level of its opening, and after passing 

 through the somatic layer of the follicle it cuts the epithelial capsule on 

 the left side at the place where this is becoming thickened to form the 

 supporting ring, 23, of the placenta. This section shows that the somatic 

 and visceral layers of the follicle are, at this level, in contact with each 

 other at each end of the median axis, and later stages show that the 

 bottom of this section is anterior and its top posterior. The section cuts 

 the body cavity four times, twice on each side in front of and behind the 

 perithoracic tubes. The two anterior divisions of the body cavity com- 

 municate at this level, but the two posterior ones do not, as the somatic 

 follicle cells and the visceral ones are united at the posterior end of the 

 middle line, as they are also around the openings of the perithoracic 

 tubes. 



The next section, Fig. 2, passes through the atrium, g"', and the body 

 cavity, 15, which is bounded within by the visceral layer of follicle cells, 

 8, and externally by the somatic layer, 7. On the left side this section 

 also cuts the top of the cavity of the placenta, y". In this and the two 

 following sections, 4 and 5, the somatic follicle cells which line the 

 atrium, g'", and the perithoracic tubes, g', are colored orange, like the 

 blastomeres, as I did not discover their follicular origin until the plate 

 had been made. 



Figure 4 cuts the cavity of the placenta, y", and the supporting ring, 

 23, and it also cuts the two perithoracic tubes, g', below the level of the 

 atrium. At the level of this section the embryonic mass is almost com- 

 pletely separated from the somatic layer, 7, and it is almost free in the 

 body cavity, 15. The next section, Fig. 5, cuts the right perithoracic tube 

 near the bottom of its lumen, g, and the left below the lumen. The 

 embryonic mass, which is free in the body cavity at this level, is divided 

 into six well-marked regions, two of them paired and four on the middle 

 line. The two paired structures are the perithoracic tubes, and each 

 consists of a thick outer wall of visceral follicle cells and an internal 

 epithelial of somatic follicle cells, colored orange in the figure. On the 

 middle line there is an elongated mass or plate of visceral follicle cells, 8, 



