190 



GENERAL PRINCIPLES OF ZOOLOGY. 



push into this jelly ; thus there arises a middle layer, the 

 mesenchyme (Fig. 103), somewhat similar to gelatinous con- 



'i^CYS.Vv ^^O / "J 



v^<sx 



- : W^ K 

 ^ cc ^ 



X^%A- 



/^ % SSM& Jil? 



\Miw<f<* 



FIG. 103. Formation of the Mesenchyme and beginning of gastrulation in Holothuria. tubu- 

 losa. (After Selenka, from Balfour.) cc, cleavage cavity ; /, ectoblast ; hy, entoblast ; 

 is, mesenchyme-cells ; ae, archenteron. 



nective tissue, from which certain organs take their origin 

 either wholly or in part. 



Mesepithelium. In the second case, the middle germ- 

 layer may preserve the epithelial character of the two- 

 primary germ-layers, so that it is called mesepithehum. 

 A />' 



FIG. 104. Formation of the mesepithelium and of the coslom of Sagitta. A. From the 

 bottom of the gastrula arise two folds, which divide the archenteron into the permanent 

 digestive tract and the coelomic diverticula. B. The separation is almost completed by 

 the pushing up of the folds, ak. outer, ik, middle, ?/t, inner germ-layer ; tnk*, somato- 

 pleure ; ink*, splanchnopleure ; //i, body-cavity. 



The mesepithelium is a part of the entoblast constricted' 

 off, the mode of development of which is shown in the 

 embryology of the worm Sagitta (Fig. 104). 



