16 



YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



ends of the cell bodies anastomose freely immediately beneath the ectoderm, but no external 

 limiting membrane is to be distinguished, such as Bardeen (1900) has described in the pig. 

 Williams (1910) in the chick finds the dermatomes exactly similar to the above and is also 

 unable to demonstrate any external limiting membrane. Regarding Bardeen's contention 

 that the dermatome does not form the cutis layer of the skin and really gives rise to muscle 

 tissue only, as he found in the pig, little light can be obtained here. It is significant, how- 

 ever, and partly confirmatory of Bardeen's results, that in the most advanced segments 

 the dermatomes are beginning to recede from contact with the ectoderm and loose mesen- 

 chyme in finding its way between the two at the edges of the dermatome. It is to be 

 expected that this loose mesenchymatous tissue would develop in situ into the cutis layer 

 later on, and, if so, the so-called dermatome would be really all myotome. 



3. Graphic reconstruction to show the position and relations of the mesodermic somites and the 



nephrogenic system in Embryo VI. 



4. Graphic reconstruction to show the position and relations of the mesodermic somites and nephrogenic 



system in Embryo V. 

 The numbers refer to the mesodermic somites. For abbreviations sec page Hi. 



The undifferentiated mesoderm back of the somites is very solid and condensed, and 

 lies in contact with the medullary canal and the ectoderm, no loose tissue whatever inter- 

 vening. The intermediate cell mass at first is cut off from it, forming the nephrogenic 

 cord, but very soon is seen to be fused with the somitic plate on one hand and the splanchnic 

 and somatic mesoderm on the other. This common mass of mesoderm passes into the tail 

 to fuse with the primitive streak. 



For some distance caudad of the last somite, a peculiar narrow process passes off ven- 

 trally from the undifferentiated mesoderm on each side and, running between the dorsal 

 aorta and the medullary groove, comes into contact with the notochord. It is very sug- 



