THE POSTBRIOE SOMITES. 91 



essentially the same manner, bat 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 tlie 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, tlie cavity of the lateral portions of the so- 

 mites becomes extremely reduced in size, in consequence of 

 the enormous thickening of their mesodermal walls (Plate Vll^ 

 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 ccelomic 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 (Plate VII, 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- 

 ing 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, Plate VIII, fig. 40, /. s. v.6^). 1 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. Plate VII, fig. 25, 

 0. s. 7, with Plate VIII, fig. 40, o. n. 6). 



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

 of tlie anterior somites are small spaces in the base of the legs 

 with a ventral prolongation which lies along the outer edge of, 

 and, except in the case of legs 4 and 5, opens to the exterior 

 immediately outside the nerve-cord. In the case of these 



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

 the contraction of the specimen. 



