172 EMBRYOLOGY. 



the origin of the connective substances ; the question of the origin* 

 of the vascular endothelium and of the blood, on the contrary, is 

 one that is less cleared up. This determines me to treat the two- 

 questions separately in the following account. 



A. The Origin of the Connective Tissues. 



Selachian embryos appear to be the most suitable objects on 

 which to trace the origin of the connective substances. Here the 

 middle germ-layer serves as the matrix for the mesenchymatic tissue. 

 At the time when the primitive segment is still connected below with 

 the lateral plates, and when the body-cavity is visible in the latter, 

 there appears a cell-growth at the lower border of each primitive 

 segment on the side which is directed toward the chorda. It is ordi- 

 narily designated as sclerotome. It contains at first a small evagi- 

 nation of the body-cavity (fig. 258 A sic). At the restricted place- 

 designated, which is marked off from its surroundings, and which 

 recurs on each primitive segment, cells in large numbers (fig. 110 

 sk) individually detach themselves from the epithelial layer, remove 

 by active migration from their place of origin, like the mesen- 

 chymatic cells of Invertebrates, and distribute themselves in th& 

 space which is limited on the one side by the inner wall (mp) 

 of the primitive segment, and on the other by the chorda (ch) 

 and the neural tube (nr). 



At the time of their appearance the ameboid cells are separated 

 by only a small amount of inter-cellular substance : they increase 

 rapidly in number, and thereby soon crowd chorda, neural tube, and 

 primitive segment farther apart (fig. 111). The segmental arrange- 

 ment which the growths exhibit at their first appearance (fig. 195 Vr) 

 very early ceases to exist, since by their extension they become fused 

 together into a continuous sheet. 



The mesenchyme, which thus grows forth out of the middle germ- 

 layer on both sides of the chorda, furnishes the foundation for the 

 whole axial skeleton ; it produces the skeletogenous tissue by the- 

 growing toward each other and the fusion of the masses which are- 

 formed on the right and left sides. As fig. Ill shows, the mesen- 

 chyme (sk) grows around the chorda (ch) both dorsally and ventrally, 

 and envelops it with a connective-tissue sheath, which is continually 

 becoming thicker. In the same manner it encloses the neural tube 

 (nr) and forms the membrana reuniens superior of the older embryo- 

 legists, the foundation out of which subsequently the connective- 



