158 



GENERAL PRINCIPLES OF ZOO LOOT. 



to gelatinous connective tissue, from which certain organs either 

 wholly or in part take their origin. 



Mesothelium. In the second case the mesoderm may preserve 

 the epithelial character of the two primary germ-layers, so that it 



FIG. 107. Formation of the mesenchyme and beginning of gastrulation in Holothuria- 

 tubulosa. (After Selenka, from Balfour.) cc, cleavage cavity; ep, ectoderm; Tiy, 

 entoderm; ms, mesenchyme cells; ae, archenteron. 



is called mesothelium. The mesothelium is cut off from the 

 entoderm, the mode of development being shown in the embryology 

 of the worm Sagitta (fig. 108). 



FIG. 108. Formation of the mesothelium and coelom 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 com- 

 pleted by the pushing up of the folds, afc, outer, mk, middle, t/c, inner germ- 

 layer; wife 1 , somatic layer; rnfc 2 , splanchnic layer; Ih, body-cavity. 



Coelomic Pouches. When the gastrula of Sagitta has been 

 formed two folds arise from the archenteric walls opposite the 

 blastopore (A), thus partially separating a pair of lateral chambers 

 from the rest. The process continues; the blastopore closes, while 



