EMBRYOLOGY 



405 



afterwards the cells behind the furrows aggregate, and a second fur- 

 row appears sharply delimiting the second pair of the somites. 

 From this time on the process of somite formation continues fairly 

 regularly, each new pair being formed behind those preceding and the 

 embryonic region grows at the expense of the primitive streak. It 

 finally comes to an end on the fifth day of incubation, by which time 

 fifty-two somites are formed. Not only is this number and order 

 of formation constant, but the fate of the individual somites is also 

 the same. The first pair of incomplete somites lie just behind the 

 region where the ear will later develop, and the next three pairs with 

 the former take part in the formation of the occipital region of the 

 head. Although in lower forms like Scy Ilium segments are laid 

 down in front of the auditory pit, this is not the case in the chick, 



MC 



CC 



M 



MC 



So 



FIG. 141. Chick. Transverse section through last somite of a 

 chick with twenty-nine pairs of somites (about forty-eight 

 hours x ). Adapted from Lillie. 



A., aorta ; C., ccelom ; C.C., canalis centralis ; E., ectoderm ; En., entoderm ; M., myotome ; 

 M.C., myoccel ; Me., mesothelium ; N., notochord ; N.C., neural crest ; N.T., nephrotome ; 

 S.C., spinal cord ; So., somatic mesoderm ; Sp., splanchnic mesoderm. 



and there is no indication of segmentation in front of the ear. The 

 mesodermal somites therefore express very clearly the primitive 

 metamerically segmented condition of the body, which is hidden 

 to a large extent in the adult. As the rate of incubation varies 

 with different conditions and individuals, it is often the custom to 

 distinguish the stage of development reached by the number of 

 somites present rather than by the number of hours of incubation. 



A transverse section shows clearly the relation between the two 

 regions of mesoderm. The outer cells of the somite are seen to be 

 arranged in a fairly definite layer, the mesothelium, around -a more 

 central mass. Between these two divisions is a very narrow 



