GENERAL EMBRYOLOGY 



1-17 



arises a third germ-layer, which owing to its position between the first 

 two, is called the mesoderm, mcsohhist, or middle germ-layer; this naturally 

 can come only from the cell material of the existing germ-layers, indeed 

 only the entoderm seems to participate in it. Two methods can be 

 distinguished in its formation. In one the space between ectoderm and 



FIG. 108. Larvae of Strongylocentrotus llvidus (after Boveri). Left a blastula with 

 mesen chyme formation; right a gastrula with differentiated mesenchyme. 



entoderm becomes widened by the secretion of gelatinous substance, and 

 from the entoderm isolated cells push into this jelly; thus there arises 

 a middle layer, the mesenchyme (fig. 108), somewhat similar to gelatinous 

 connective tissue, from which certain organs either wholly or in part 

 take their origin. The mesenchyme can be formed before the beginning of 

 gastrulation or after that process has been completed (fig. 108; cf. p. 74). 



FIG. 109. Formation of the mesothelium and cu'lom of Sagidti. A. From the 

 bottom of the gastrula arise two folds, which divide the archenteron into the perm;uu nt 

 digestive tract and the ccelomic diverticula. B. The separation is almo-t c<. m plrird 

 by the pushing up of the folds, ak, outer, ink, middle, //,', inner germ-layer; >nk\ 

 somatic layer; mk~, splanchnic layer; Ih, body-cavity. 



Mesothelium. In the second case the mesoderm may preserve the 

 epithelial character' of the two primary germ-layers, and is called m< 

 thdiitm. It is cut off from the entoderm, the mode of development being 

 shown in the worm Sagitta (fig. 109). When the gastrula of Sii : ^itta 

 has been formed two folds arise from the archenteric walls opposite the 



