158 



OENBHAL PRINCIPLES OF ZOOLOGY. 



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



Fig, 107.— Formation of the mesencliyme and beginning of gastmlation in Holofhnn'a 

 tiihuUisd. (After 8elenka, from Balfour.) cf, cleavage caviti^; ep, ectoderto; hy, 

 entoderm; ;/(.s, mesencliyme cells; ae, archenteron. 



is called mesoilieliuin. The mesothelium is cut off from the 

 entoderm, the mode of development being shown in the embryology 

 of the worm Sagitlii (fig. 108). 



A n 



Fig. lO.S.— F )rm ition of the Tin sr tlu Inim and radoni of Sn<titia. A. From tlie hottom 

 of the ^ o^triil I iiise two tol K ^^llI h di\'ide the archenteron into the permanent 

 diget^tn ( tl ict andthecalomn duerticula. B. The separation is almost com- 

 pleted bs the pushing up of tlie folds, ale, outer, »i)r, mid<lle, ih\ inner germ- 

 layer; iMfc', somatic layer; ink'', splanchnic layer; Vi, body-cavity. 



Coelomic Pouches. — AMicii the ga.strula of Sni/i/fa h;is licen 

 formed two folds arise from the archenteric walls ojiposito the 

 bliistoporo (^1), thus jiartially separating a pair of lateral chambers 

 from tlic rest. The jirorcss continues; tlie blastopore closes, Avhile 



