SPLITTING OF MESODEIOI 



41 



by the splitting apart of its floor. Such a process actually takes place 

 in reptiles, and in an abbreviated fashion in the bats among mammals (figs. 67 

 and 68, p. 4~>). 



Looking back to the complicated series of changes described above, we may divide the 

 period of the appearance of the mesoderm into two stages. In the first the mesoderm of the 



FIG. 60. MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF TABSIUS SOMEWHAT LATER 



THAN THAT SHOWN IN FIG. 57. (After Hubrecht.) 



p.A;.,.Hensen's (protochordal) knot continued backwards into the thickening of the primitive streak ; 

 g.d., germinal disc ; ect, ectoderm of somatopleure ; mes 1 , mesoderm of ditto ; coe, cceloni ; p.p., primitive 

 entodermic i protochordal) plate, continuous with _p.^r., protochordal process ; ent, entoderm; all, rudi- 

 ment of allaiitoic diverticulum ; mes 2 , mesoderm of splanchnopleure. 



connecting stalk (yolk-sac and amnion) and the head-plate is laid down. It is a continuous 

 and unpaired sheet, and is at no time segmented. In the second phase the primitive streak 

 appears, and the mesoderm now springs from its sides as bilateral sheets ; the embryonic axis 

 is differentiated out of the proliferating streak-tissue, the notochordal process and plate take 

 form, and from the sides of the plate the mesoderm continues to be thrown off as the dorsal 

 segmented mesoderm of the embryonic body. 



per 



FIG. 61. MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF TARSIUS AT A 



LATER STAGE THAN THAT SHOWN IN THE LAST FIGURE. (After Hubrecht.) 



n.c.. neurenteric canal ; p.p., primitive entodermic (protochordal) plate ; 

 p-pr., notochordal plate ; am 1 anterior, air? posterior amnion fold ; all, allantoic 

 diverticulum ; per, pericardial ccelom. 



At the period now reached the blastoderm has the appearance presented in 

 fig. 63. Sections demonstrate that the mesoderm everywhere forms a continuous 

 layer. Within the shield it is still undivided, but outside the shield it spreads as 

 two lamellae from the edge of the disc, the somatopleuric lamella surrounding the 

 amnion and lining the wall of the blastocyst, and the splanchnopleuric enveloping 

 the yolk-sac. 



