508 ADAM SEDGWICK. 



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

 embryo is concerned, quite distinct from the cavity of the 

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

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

 essentially the same manner, but not at the same stage ; the 

 coelomic spaces arise as vacuoles in the multinucleated bodies 

 called mesoblastic somites, while the parietal spaces arise later 

 as the result of the vacuolation of multinucleated masses de- 

 rived from the walls of the somites. 



The development so far has only differed from that of the 

 third somite in the much earlier separation of the median from 

 the lateral portion of the somite. 



In Stage e, the cavity of the lateral portions of the so- 

 mites becomes extremely reduced in size, in consequence of 

 the enormous thickening of their mesodermal walls (PL XXXV, 

 fig. 25, l.s. 7), and at the same time confined to the base of 

 the appendage, the whole of the distal part of the latter being 

 occupied by a mass of mesoderm cells. 



By the end of Stage e, the coelomic space of the fourth leg 



(seventh segment) has acquired an opening to the exterior in 



nearly the same position as the opening of the third somite, i. e. 



immediately external to the nerve-cord, and by the same process, 



viz. a ventral outgrowth from the coelom, which meets and fuses 



with the ectoderm (PI. XXXV, fig. 25). The same process 



takes place in the three preceding legs (legs 1 — 3), I think, a 



little later. If this is so, we have an exception to the prevail- 



ino- rule of development from before backwards. However 



this may be, the three preceding somites obviously possess 



their opening at a slightly later stage (early embryo of Stage 



F, PI. XXXVI, fig. 40, /. s.v.6^). I should mention that even at 



this early stage the external openings of somites 7 and 8 



have not the same position as in the case of the other legs, but 



are nearer the periphery of the limb (cf. PI. XXXV, fig. 25, 



0. s. 7, with PI. XXXVI, fig. 40, o. n. 6). 



To sum up, at the beginning of Stage f the lateral portions 



^ The apparent absence of a lumen in the passage in this figure is due to 

 the contraction of the specimen. 



