90 THE CHANGKS FROM STAGE A TO F. 



of a nephridium of the jaw somite, unless, as Kennel has 

 suggested, the internal prolongation of the jaw be regarded as 

 such. 



The Somites of the Fourth to the Fifteenth Pairs, i. e. of the first 

 to the twelfth legs, may be taken together. The development is 

 essentially the same for all, and until Stage f almost exactly 

 the same. The description of the changes of any one of them 

 will therefore serve for all. Like the other parts of the 

 body, they develope in order from before backwards. The 

 dates given in the description below will refer to the anterior 

 somites. 



The changes during Stage d are similar to those of the third 

 somite, and the figures 10 — 12, Plate VI, which were used to 

 illustrate my description of the latter during Stage d, are 

 really representations of these posterior somites. As a result 

 of these changes the thickening of the somatic wall, the leg diver- 

 ticulum, and the rudimentary septum, which partly separates 

 the latter from the rest of the somites, are established. In 

 Stage E, the somite has become divided into two parts by the 

 completion of the septum, — into an anterior part placed dor- 

 sally (fig. 24, S. 4) and a posterior part with a very small lumen 

 contained in the leg (Plate VII, fig. 25 L s. 7). These two 

 portions do not overlap, as might be imagined from the earlier 

 stage seen in figs.^ 13 or 21 c, in which the septum is incomplete, 

 the part of the somite in the body on a level with the leg 

 having disappeared. The spaces in the parietal mass of me- 

 soderm, the origin and history of which has already been 

 described, have increased largely in size (fig. 25) and might 

 easily be mistaken for a portion of the true coelom. It must 

 be carefully borne in mind — as I have already pointed out — 

 that these spaces are, so far as their development in this 

 embryo is concerned, quite distinct from tlie cavity of the 

 somites, which I regard as the true coelom. I may, however, 

 draw attention to the fact that the two sets of spaces arise in 



' These figures were not taken from the parts here referred to, but inasmuch 

 as they precisely resemble in all particulars sections from these parts, they 

 can be used as illustrations of the text. 



