AMPHIBIA. 



123 



of the mesenteron 1 , and (2) to the enclosure of the yolk-cells by 

 the epiblast. 



The mesenteron is formed as in Petromyzon and Lepidosteus 

 by an unsymmetrical form of 

 invagination. The invagina- 

 tion first commences by an in- 

 flection of the epiblast-cells for 

 a small arc on the equatorial 

 line which marks the junction 

 between the epiblastic cells and 

 the yolk-cells (fig. 70, x). 



The inflected cells become 

 continuous with the adjoining 

 cells ; and the region where 

 the inflection is formed consti- 

 tutes a kind of lip, below which 

 a slit-like cavity is soon es- 

 tablished. This lip is equiva- 

 lent to the embryonic rim of 

 the Elasmobranch blastoderm, 

 and the cavity beneath it is the 

 rudiment of the mesenteron: 



The mesenteron now rapidly extends by the invagination of 

 the cells on its dorsal side. These cells grow inwards towards 

 the segmentation cavity as a layer of cells several rows deep. 

 At its inner end, this layer is continuous with the yolk-cells ; 

 and is divided into two strata (fig. 71 A), viz. (i) a stratum of 

 several rows of cells adjoining the epiblast, which becomes the 

 mesoblast (m), and (2) a stratum of a single row of more 

 columnar cells lining the cavity of the mesenteron, which forms 

 the hypoblast (hy\ The growth inwards of the dorsal wall of 

 the mesenteron is no doubt in part a true invagination, but it 

 seems probable that it is also due in a large measure to an actual 

 differentiation of yolk-cells along the line of growth. The 

 mesenteron is at first a simple slit between the yolk and the 

 hypoblast (fig. 71 A), but as the involution of the hypoblast and 



1 Since the body cavity is not developed as diverticula from the cavity of invagina- 

 tion, the latter cavity may conveniently be called the mesenteron and not the archen- 

 teron. 



FIG. 70. SECTION THROUGH FROG'S 

 OVUM AT THE CLOSE OF SEGMENTATION. 

 (After Gotte.) 



sg. segmentation cavity ; //. large yolk- 

 containing cells ; ep. small cells at forma- 

 tive pole (epiblast); or. point of inflection 

 of epiblast ; y. small cells close to junction 

 of the epiblast and yolk. 



