696 EMILY RAY GREGORY, 



A Study of Platypeltis embryo XII (3), and of the following stages, 

 indicates very clearly that the secondary and later series of tubules 

 arise in situ from part of the original blastema. The beginnings of 

 the secondary tubules can be easily followed and their independent 

 character demonstrated. In this embryo there still remains consider- 

 able blastemic tissue around the tubules in the kidney lobe. In 

 stage XVI, however, on the outer and lower sides of the lobe we find 

 only the large primary and secondary tubules with their glomeruli, 

 with large irregular spaces lined by a single attenuated layer of endo- 

 thelial cells. Frequently a few blood corpuscles may be seen in these 

 spaces. On the upper and inner side of the lobe, however, we still 

 find a dense mass of blastemic cells from which tubules and glomeruli 

 are still developing. I am not disposed to say that tubules never 

 branch but it appears rather from the above observations, that they 

 arise independently, but in the course of growth may come into con- 

 tact and fuse with other tubules. In stage XIX the cells of the lower 

 part of the tubules have increased greatly in size and those of the 

 lower and second curve are enormous compared with their original 

 size. In most of them the large nucleus is clearly visible and some 

 are filled with densely staining granules. The cells of the duct have 

 grown comparatively little. This development seems always to begin 

 at the lower end and to extend gradually upwards. The kidney lobe 

 is large and its wall is composed of extremely flat, thread-hke cells. 



Glomerulus. 



The blood supply of the first ten somites has been discussed 

 under the head of pronephros and we may now consider more in 

 detail the conditions which obtain further back. As mentioned be- 

 fore, in IV (1) in the tenth somite and for a short distance further, 

 we find a vascular tuft within the tubule which we must consider the 

 first appearance of the glomerulus. The first trace of this tuft is 

 seen in a tubule at the end of somite nine before the lacunae of 

 the glomus have disappeared. The two conditions, therefore, overlap. 

 On the dorsal side of the median end of the tubule there is a thick 

 mass of dense staining cells which extends into the lumen of the 

 tubule as a projecting tuft. The cells are less dense at the outer 

 edge and are seen to be very round and loosely held together, though 

 there are no spaces. In some tubules the tuft looks like three or 

 four bunches of grapes in a cluster. In most of the embryos of about 

 this age, however, this tuft is not found regularly on the median 



