694 EMILY RAY GREGORY, 



The pronephric region ends with the tenth somite where we 

 sometimes find the two overlapping elements clearly differentiated, 

 therefore, we have in somite XI the beginning of the purely 

 mesonephric part. This extends through the body to the opening of 

 the Wolffian duct into the cloaca. In stage II (with perhaps one ex- 

 ception where the end is injured) the division into somites has not 

 yet been completed and the duct does not reach the cloaca. Here 

 the mesonephric basis may be distinguished as far as the somatic 

 division extends and perhaps a little beyond, by the thickening of the 

 middle plate and the large number of mitotic figures in it. In stage III 

 the duct has reached the cloaca and the mesonephric blastema is 

 clearly laid down to the end. There are no tubules with a lumen 

 opening into the duct beyond the tenth somite, and from this point 

 on, the blastema extends as a rod of concentric, densely packed cells, 

 its round shape modified from point to point where the tubules are 

 growing towards the duct. In these places there is often a small 

 lumen, especially near the anterior end. In each of somites eleven and 

 twelve of III (5) there seem to be two tubules developing and only 

 the blastema beyond. In the posterior part of III (4) I also noted 

 two tubules to a somite. In IV (1), however, which is more advanced, 

 I counted ten on one side and on the other thirteen more or less 

 rudimentary tubules beyond the pronephric region, but the arrange- 

 ment was very irregular. The tubules continue to develop successively 

 backwards and the continuity of the blastema as well as its original 

 shape disappears. But the growth of the skeleton and other organs 

 cramps the posterior end so that the tubules cannot develop and 

 here for a short distance there remain only the solid rudiments 

 of tubules and perhaps finally only a very small irregular, rod-like 

 blastema. 



Passing over, at least for the present, stages VII, VIII, IX and 

 X, we come to XI and XII. Here we seem to have a regularly de- 

 veloped stage of the mcsonephros but one not yet very complicated 

 in its structure. The number of the tubules in a somite is variable. 

 Taking Flatypeltis^ XII (3), of which the pronephric region is found 

 on Plate 47, Figs. 52—56 for our type, we find the following conditions: 

 In the eleventh somite there are five tubules on the right, and four 

 on the left; in the twelfth, thirteenth and fourteenth, three on each 

 side. In these somites, we find the form of tubule which has been 

 typical up to this stage. On the right it is an S spreading into a 

 funnel at its lower end to receive the glomerulus, and opening into 



