352 OUTLINES OF CHORD ATE DEVELOPMENT 



(about the tenth day) as the pleuro-peritoneal membrane, 

 closing off the pleural cavity from the body cavity proper or 

 peritoneal cavity. 



VI. THE LATER HISTORY OF THE MESODERMAL 



SOMITES 



In the chick of thirty hours we saw how the embryonic 

 mesoderm is divided into three general regions, (a) the axial 

 somites, (b) the intermediate cell mass or nephrotome, (c) the 

 distal lateral plate, continuous with the extra-embryonic meso- 

 derm (Fig. 102). We have already described the chief struc- 

 tures derived from the lateral plate the vascular system and 

 the ccelom and its derivatives, and it remains now to describe 

 the structures derived from the somites and intermediate cell 

 mass. 



The following table, quoted from Lillie (Development of the 

 Chick, pp. 184-185) gives a resume of the general disposition of 

 the somites. 



"In an embryo of 42 somites (about ninety-six hours), the value of 

 the somites as determined by their relations and subsequent history is 

 as follows : 



1 to 4. Cephalic; entering into the composition of the occipital re- 

 gion of the skull. 



5 to 16. Prebrachial; i.e., entering into the region between the wing 

 and the skull. 



17 to 19. Brachial. 



20 to 25. Between wing and leg. 



26 to 32. Leg somites. 



33 to 35. Region of cloaca. 



36 to 42. Caudal. 



" More somites are formed later, the maximum number recorded being 

 52 (see Keibel and Abraham, Normaltafeln) . In an eight-day chick 

 the number of somites is again about 42, including the four fused with 

 the skull. Thus the ten somites formed last are again lost." 



The somites, excepting those at each end of the series, have 

 essentially a similar history, differing only in later details of 

 development (Fig. 119). Each gives rise to three structures: 

 (a) the muscle plate or myotome, (b) the cutis plate or dermatome, 



