174 EMBRYOLOGY. 



continuous epithelium. Then a part of the wall of the primitive 

 segment lying at its lower and median angle begins to grow with 

 extraordinary rapidity, and to furnish a mass of embryonic connective 

 tissue, which spreads itself around the chorda and neural tube in the 

 manner previously described. The dorsal and lateral parts of the 

 primitive segment (fig. 116 ms), which subsequently loses its cavity, 

 are not involved in this growth ; out of them arise principally the 

 fundaments of the trunk-musculature. This part is consequently 

 now distinguished as muscle-plate (ms). 



Mesenchyme arises from three other places of the middle germ- 

 layer besides the primitive segments from the visceral lamella, from 

 the parietal" lamella, and finally from that wall of the primitive 

 segment which is turned toward the epidermis and has been given 

 by RABL the name cutis-plate. Here also the conditions are best 

 followed in Selachii. 



Individual cells migrate out from the visceral lamella (Darm- 

 faserblatt), which in early stages is composed partly of cubical, 

 .partly of cylindrical cells (fig. 110 m& 2 ), and distribute themselves 

 upon the surface of the entodermic layer ; they are found at places 

 where no trace of a vessel is observable. They furnish the 

 mesenchyma of the intestinal wall, which is ever becoming more 

 abundant, and which is subsequently converted partly into connective 

 tissue, partly into the smooth muscle-cells of the tunica muscularis 

 (fig. Ill mes*). 



A similar process is repeated in the parietal lamella (Haut- 

 faserblatt). Emigrating cells produce between the epithelium of 

 the body-cavity and that of the epidermis an intermediate layer of 

 mesenchyme-cells (fig. 110 ink 1 , fig. Ill mes 1 ). 



An important region for the production of connective tissue is, 

 finally, the cutis-plate, i.e., the epithelial layer of the original primi- 

 tive segment which is in contact with the epidermis (fig. 110 cp). 

 The process occurs here later than at the other places mentioned, 

 and begins with an active cell-growth, which gradually leads to a 

 complete disintegration of the epithelial lamella. " The disintegra- 

 tion," as EABL remarks, " proceeds in such a manner that the cells, 

 which hitherto exhibited an epithelial character, separate them- 

 selves from one another, and thereby lose their epithelial character." 

 It is probably from this part of the mesenchyme that the cerium is 

 derived. 



That the mesenchyme-cells scattered between the epithelial lam- 

 ellae are capable of executing extensive migrations, after the fashion 



