AMPHIBIA. 



129 



a vertebral portion, cleft into separate somites, and a lateral un- 

 segmented portion (fig. 74). 



The history of these two parts and of the mesoblast is 

 generally the same as in Elasmobranchs. 



The mesoblast in the head becomes, according to Gotte, divided into 

 four segments, equivalent to the trunk somites. Owing to a confusion into 

 which Gotte has fallen from not recognizing the epiblastic origin of the 

 cranial nerves, his statements on this head must, I think, be accepted with 

 considerable reserve ; but some part of his segments appears to correspond 

 with the head-cavities of Elasmobranchii. 



Urodela. Scott and Osborn (No. 114) have shewn that in the Newt 

 the mesoblast (fig. 75) is formed of two lateral plates, split off from the 

 hypoblast, and that^the ventral growth of these plates is largely effected by 

 the conversion of yolk-cells into mesoblast-cells. They have further shewn 

 that the notochord is formed of an axial portion of the hypoblast, as in the 

 types already considered (fig. 75). The body cavity is continued into the 

 region of the head ; and the mesoblast lining the cephalic section of the 

 body cavity is divided into the same number of head cavities as in Elasmo- 

 branchii, viz. one in front of the mouth, and one in the mandibular and one 

 in each of the following arches. 



The hypoblast. There are no important points of difference 

 in the relations of the hypoblast between the Anura and 

 Urodela. The mesente- 

 ron, at the stage repre- 

 sented in fig. 73, forms a 

 wide cavity lined dorsally 

 by a layer of invaginated 

 hypoblast, and ventrally 

 by the yolk-cells. The 

 hypoblast is continuous 

 laterally and in front with 

 the yolk-cells (figs. 72, 

 74 and 75). At an earlier 

 stage, when the mesen- 

 teron has a less definite 

 form, such a continuity 

 between the true hypo- 

 blast and the yolk-cells does not exist at the sides of the cavity. 



The definite closing in of the mesenteron by the true hypo- 

 blast-cells commences in front and behind, and takes place last 

 B. in. 9 



0.1 



FIG. 75. TRANSVERSE SECTION THROUGH 

 THE CEPHALIC REGION OF A YOUNG NEWT EM- 

 BBYO. (After Scott and Osborn.) 



In.hy. invaginated hypoblast, the dorsal part 

 of which will form the notochord ; ep. epiblast 

 of neural plate; sp. splanchnopleure ; al. ali- 

 mentary tract ; yk. and Y.hy. yolk-cells. 



