500 ADAM SEDaWICK. 



In the next stage (Stage e, PI. XXXV, figs. 23 a—c), tlie two 

 median cavities, b.h. and 5. 6c., present but little alteration, 

 excepting that the dorsal one b. h. has been encroached upon by 

 the dorsal extension of the median division of the somite. 



The successive lateral spaces have now become continuous, 

 extending through the region of the appendage immediately 

 within the septum [v. s.). This is shown in Pi. XXXV, 

 fig. 23 c—e, and more clearly by PI. XXXV, fig. 25. (The 

 tongue of cells in this space in figs. 23 a,b,l shall refer to 

 later in describing the jaw somite.) 



Before proceeding, I may mention the fate of the three 

 divisions of the pseudocoele or permanent body cavity which 

 have so far appeared. The dorsal median cavity {b. h.), which 

 is from the first a continuous space, begins at the very front 

 end of the body (PL XXXV, fig. '^2 a, b.h.) and' extends 

 backwards as far as the ectoderm has separated from the endo- 

 derm. It eventually reaches the hind end of the body, and 

 becomes, i. e. all of it, except its very front and hind ends, the 

 heart. The ventral [b. be), which extends forwards as far as 

 the mouth (PI, XXXVI, fig. 31), will form the ventral portion 

 of the median chamber of the body cavity of the adult. The 

 lateral cavity, which is at first not a continuous cavity, but 

 eventually becomes so, gives rise in the adult to the lateral 

 chamber of the body cavity (lateral sinus), which contains the 

 nerve-cord and salivary gland. 



In Stage e, two new cavities appear amongst the nuclei of 

 the ventral corners of the somites (PI. XXXV, fig. 25, b.pc. 

 and b. be'.). They are first seen at about the level of the seventh 

 somite, and soon (in Stage f) increase in size and extend forwards 

 to the level of the jaw somite, and backwards, eventually reaching 

 the hind end of the body. PI. XXXVI, fig. 39, shows the 

 typical arrangement of these two additional cavities in Stage f. 

 The dorsal of them (b.pc.) has extended dorsalwards as far as 

 the median dorsal pseudocoele or rudimentary heart (b. /«.), which 

 has in this stage become much smaller. The ventral one {b. be'.) 

 also has extended dorsalwards as far as the ventral wall of the 

 much reduced somite (d. s.). The somite, which in the previous 



