W. K. BROOKS ON THE GENUS SALPA. 43 



period in the history of the embryo some of the visceral follicular nuclei 

 wander in among the blastomeres, or push into their substance, as is 

 shown in Plate XLII, Fig. 1, and there degenerate, as has already been 

 described. 



The stage which is shown in Plate XII and in cut B is the starting 

 point for a description of the fate of the others, since the follicle is at this 

 time most fully developed, and consists, as already described, of a somatic 

 layer, 7, which forms the outer wall of the embryo ; a visceral mass, 8, 

 and the somatic lining of the perithoracic structures, g, g', g", g'" and g". 



The somatic layer of the follicle is divisible into two parts, the upper 

 portion, cut A, 7, which is covered by the epithelial capsule, B', and the 

 lower part, 10, which is exposed to the body cavity, y, of the chain-salpa. 

 These two regions are so different in their history that they must be 

 treated separately. The lower portion, 10, forms the roof of the placenta, 

 cuts A and B, 10, and, as is shown in cuts C and D, it soon loses its 

 continuity with the upper portion, and establishes a union with the upper 

 edge of the supporting ring, 23, which, as I have shown, itself loses its 

 continuity with the epithelial capsule, B', and bends inwards, as shown 

 in cut C, to unite with the follicular roof of the placenta. In Plate XII, 

 the cavity of the placenta, colored yellow, is shown at y" in Figs. 2, 4, 5, 6, 

 7, 8 and 9, and the supporting ring, 23, which is colored red, is shown in 

 the same figures, where the part of the somatic layer of the follicle which 

 is to enter into the formation of the roof of the placenta is marked 10. 

 The relations between these various structures are so well shown in the 

 figures and in the reconstructed cross section in cut B that description 

 seems unnecessary, but we must note that the upper edge of the placenta 

 is not a flat circle, but that it reaches much farther up on the sides of the 

 embryo than it does on the middle line, and that the line which separates 

 the placental portion of the somatic layer from the embryonic portion 

 follows the same course, as does also the zone of transition from the 

 epithelial capsule to the supporting ring. Comparison of cuts A, B, C 

 and D also shows that as the embryo grows, the roof of the placenta 

 becomes flat, and that the embryo itself pushes upwards from the surface 

 which is thus formed. 



The fate of the follicular cells of the roof of the placenta will be 

 described in the account of the placenta, and we have now to consider 

 the fate of those follicle cells which are more intimately bound up in the 

 structure of the embryo. 



