686 EMILY RAY GREGORY, 



anterior and posterior limits, respectively, of the organ. In only one 

 case have I found clear indications of the pronephric basis between 

 the third and fourth somites and a careful study of that embryo in- 

 dicates that the segmentation towards the head has been retarded 

 and that these somites, therefore, correspond to the fourth and fifth 

 in other embryos. In none have I found the basis extending beyond 

 the tenth somite. The individual embryos vary considerably as to 

 the position of the anterior end of the basis, which on one side (it may 

 be either one) almost invariably extends farther forward than on the 

 other. 



In Aromochel/ys II (3) the first slight swelling appears on one 

 side at the end of the fourth somite. At the end of the fifth somite 

 it is less distinct and only at the sixth is it pronounced on both sides. 

 From this point it forms a conspicuous welt. The drawings of this 

 embryo on Plate 45, Figs. 1—14, will make the condition quite plain. 

 The first section of somite VI (Fig. 1) shows the very edge of the 

 somite. There is no sign of any swelling on the somite and the side 

 plates are quite flat. Section 5 of the same somite (Fig. 2) shows a 

 wider connection between somite and middle plate. The upper part 

 of this is due to the proliferation, from the somite, of cells for the 

 pronephric basis. Two sections further back we see that the welt is 

 a conspicuous feature, and by internal growth pushes up and against 

 the ectoderm. Section 8 (Fig. 3) is near the end of the somite, but 

 the ridge remains and even increases. Sections 9 and 10 (Figs. 4 and 

 5) show how this welt bridges over the space between the two 

 somites, and beneath, as in two sections before, the cells which form 

 the walls of the aorta can be seen shpping out from the splanchnic 

 mesoderm and lying beneath the somite. In section 6 of somite VII 

 (Fig. 6) we see the pronephric welt reenforced, for in this case the 

 basis is still fused with the somite throughout. In section 10 (Fig. 8) 

 we see the same bridge as before, although the right side is a little 

 lower. The sixth section of somite VIII (Fig. 9) shows again most 

 clearly how the basis of the pronephros is a direct outgrowth from 

 the somite. The arrangement of the cells makes it still a part of 

 the somite. Here, too, on the left, we see that this solid extension 

 from the somite is gaining a lumen which will open into the coelom 

 beyond. From this point the ridge gradually lowers, though we still 

 clearly see it as a rudiment in the tenth somite. Beyond this the 

 embryo is unsegmented, and on the right the last sign of the pro- 

 nephric welt is seen. Section 10 of somite VI (Fig. 18) really lying 



