EXPLANATION OF PLATES. 379 



Figure 3, a section through an egg after the first division, showing one of the two 

 primary blastomeres about to divide the second time. 5 is the primary cavity of the 

 follicle. 8 marks the point where the proliferation and migration of the follicle cells 

 first take place. 7, the thickened and proliferating portion of the follicle. 10, the 

 unmodified portion of the follicle. 13, pole-cells. 



Figures 4 and 5, two sections from a series through an egg a little older than the 

 one shown in Plate IX, to show the proliferation and migration of the follicle cells. 

 5, the primary cavity of the follicle. 7, the somatic layer of the follicle. 8, follicle 

 cells migrating inwards to form the visceral layer of the follicle. 9, the blastomeres. 

 10, the area from which the follicle cells push in among the blastomeres. 6, the wall 

 of the cloaca, c, the cavity of the cloaca. 



A section of another egg at nearly the same stage, but a little younger, is shown 

 in Plate IX, Fig. 11. In this egg the migrating area of the follicle showed a distinct 

 -invagination at 6. I was not able to find this invagination in other eggs. 



Figures 6, 7 and 8, three sections through an egg a little older than the one shown 

 in Figures 4 and 5. The area 10 is now bounded by definite limits, and its cells are 

 beginning to migrate inwards among the blastomeres. 



Figure 9, a longitudinal section through an embryo at the stage shown in Plate 

 XI, Figs. 1 and 2. The primary follicular cavity has been obliterated by the migrat- 

 ing follicle cells, and the somatic layer 7 of the follicle is in contact with the splanch- 

 nic layer 8. At 10 the two layers are continuous with each other. This embryo is 

 the one which is shown at D in Fig. 10. 



Figure 10, a series of four eggs and embryo at successive stages of development, 

 from Salpa hexagona. A, an unfertilized egg ; B, an egg at the stage shown in Fig. 2; 

 C, an embryo at the stage shown in Fig. 8 ; D, an embryo at the stage shown in Fig. 9 ; 

 b, the thickened epithelium of the wall of the cloaca ; x, the fertilizing duct ; y, the 

 body cavity ; 11, the secondary connection between the cloaca and the fertilizing duct. 



PLATE XL 

 SECTIONS OF YOUNG EMBRYOS OF SALPA HEXAGONA. 



c, in Figs. 1, 2, 3, 4, 5 and 6, the cavity of the cloaca of the chain-salpa. In Figs. 

 3, 7 and 8, the letter c, without a reference line, also marks the cloaca of the chain- 

 salpa, while the c with a reference line marks the branchial sac of the embryo, q, the 

 place where the mouth is to be formed, t, the rudimentary ciliated funnel or sub- 

 neural gland, x, fertilizing duct, y, body cavity of chain-salpa. 7, somatic layer 

 of follicle. 8, splanchnic layer of follicle. 9, blastomeres. 9', ectodermal blasto- 

 meres. 15, secondary cavity of follicle or body cavity of embryo. 



Figures 4, 5, 6, 7 and 8 are horizontal sections from the same embryo. Fig. 4 is 

 the sixteenth section in the series ; Fig. 5 is the eighteenth ; Fig. 6 is the twenty- 

 fourth ; Fig. 7 is the twenty-eighth ; Fig. 8 is the thirty-fifth. 



PLATE XII. 

 HORIZONTAL SECTION THROUGH THE YOUNGEST ONE OF MY EMBRYOS OF SALPA 



PlNNATA. 



Figure 5 of Plate XVII and Figs. 1 to 9 of this plate form a series, of which Plate 

 XVII, Fig. 5, is the sixteenth section ; Plate XII, Fig. 1, the twentieth ; Fig. 2, the 

 twenty-fourth ; Fig. 3, the twenty-sixth ; Fig. 4, the thirty-first ; Fig. 5, the thirty- 

 sixth ; Fig. 6, the fortieth ; Fig. 7, the forty-sixth ; Fig. 8, the fiftieth, and Fig. 9, the 

 sixtieth. Cut B, on page 29, is a reconstruction, from this series, of a vertical trans- 

 verse section, and the planes of the actual sections which are figured in this plate are 



