14 



YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



same stage of development as the corresponding somite of a chick embryo of the 18-somite 

 stage described by Williams. The dermatome is a flattened plate underlying the ectoderm 

 and formed of the whole dorsolateral wall. The myotome is flexed under the dermatome 

 medially and extends out about half the extent of the dermatome laterally, lying in con- 

 tact with the latter and including between itself and the latter a small cut-off portion of 

 the general cavity of the somite in the dorsomedial angle. 



There is a deep cleft, named by Williams the lower myotomic groove, separating the 

 myotome from the sclerotome below it. The sclerotome is formed of a mass of looser cells 

 forming the ventromedial and ventral part of the wall of the somite, and from the ventral 

 surface of this two long, curved, plate-like processes pass off. The inner one, the noto- 

 chordal process, passes medially between the medullary canal and the dorsal aorta to the 

 side of the notochord. The lateral one, the aortic process, passes ventrally around the 

 lateral surface of the aorta toward the wall of the pharynx, its ventral end being indistinct 

 and hard to differentiate from the surrounding mesoderm. The notochordal process 

 arises from the whole extent of the ventral surface of the sclerotome, but the aortic process 

 comes only from the anterior two-thirds, and there is a distinct interval between it and the 

 next succeeding aortic process, except in the case of the second and third somites, in which 

 these processes are fused. The cavity of this somite is distinct and irregular and is not 

 very large. The core is fused with the sclerotome and is indistinct. 



U. M. G. 



1. Transverse section through the middle of the fifth mesodermic somite on the right side in 



Embryo VI. Drawn with camera lucida. Magnification X 270. 



2. Transverse section through the middle of the twelfth mesodermic somite on the right side 



in Embryo VI. Drawn with camera lucida. Magnification X 270. 

 For list of abbreviations see page 40. 



The appearance of each succeeding somite differs somewhat from the description 

 given above for the second, the main difference being that proceeding caudad there is a 

 retrogression in the amount of differentiation of the parts of the somite. The size of the 

 somitic cavity increases greatly and is very large from the eighth to sixteenth somites and 

 then decreases rapidly and is merely a slit in the nineteenth. There is apparently connec- 

 tion of some of these cavities with the ccelom. 



Only the myotomes of the second to fifth somites (text fig. 1) inclusive are flexed 

 sufficiently to lie in contact with the under surface of the dermatome and so cut off a small 

 portion of the somitic cavity. Proceeding caudally, they become less and less strongly 

 flexed and the groove on the inner surface at the dorsal angle of the somite, named by 



