THE SECOND SOMITE. 89 



separated from the ectoderm ventrally and on each side of the 

 dorsal line, fig. 9) the rudiment of the jaw is laid down, and 

 the somite is prolonged into it. (Plate VI, fig. 9, which, 

 however, is through the oral papilla, represents quite accurately 

 a section through the jaws at this stage.) The somatic wall of 

 the somite becomes considerably thickened, particularly the 

 ventral portion of it. This is very marked in Stage e (fig. 20, 

 m. t.). The somite of the jaw segment is further in Stage e 

 overlapped on the dorsal side by the somite behind (fig. 20). 

 By the close of Stage e (stage of fig. 35, Plate II) the portion 

 of the cavity of the somite contained in the jaw has become 

 almost obliterated by the growth of the cells above mentioned. 

 The median portion persists for some time, and furnishes, from 

 its splanchnic wall, cells which apply themselves to the develop- 

 ing stomodseum and assist in forming the pharyngeal and 

 oesophageal musculature. 



The median division of the jaw somite, which has from 

 an early period a much less longitudinal extension than the 

 median divisions of the other somites, coalesces in Stage f 

 with the median division of the third somite. 



The walls of the ventral portion of the somite, which entirely 

 fill up the jaw, form the muscles of the jaw and are prolonged 

 backwards in the lateral compartment^ of the body cavity as 

 the tongue of cells, which has been already referred to and is 

 shown in Plate VII, fig. 23 a — c, at m. I. 



In the following Stage (f) the hind end of the jaws becomes 

 enclosed by folds of the dorso-lateral walls of the buccal cavity 

 and constitutes the rudiment (Plate VIII, fig. 36, le.) of the in- 

 ternal backward continuation of the inner blade — the so-called 

 lever of the jaw, which is so marked a feature in the adult. The 

 further history of this structure, as well as that of the tongue 

 of cells, which have been already described for Stage e and 

 are shown in sections of Stage f at m. I., Plate VIII, figs. 36 — 

 39, will be described later. 



I have not been able to see any trace of even the rudiment 



* Tlie posterior end of this tongue of cells lies in the central compartment of 

 the body cavity. 



