REPTILIA. 



203 



a layer below this several rows deep. Below this layer fresh 

 segments continue for some time to be added to the blastoderm 

 from the subjacent yolk. 



The blastoderm, which is thickened at its edge, spreads 

 rapidly over the yolk. Shortly before the yolk is half enclosed 

 a small embryonic shield (area pellucida) makes its appearance 

 near the centre of the blastoderm. The embryonic shield is 

 mainly distinguished from the re- 

 mainder of the blastoderm by the 

 more columnar character of its con- 

 stituent epiblast cells. It is some- 

 what pyriform in shape, the narrower 

 end corresponding with the future 

 posterior end of the embryo. At 

 the hind end of the shield a some- 

 what triangular primitive streak is 

 formed, consisting of epiblast con- 

 tinuous below with a great mass of 

 rounded mesoblast cells, probably 

 mainly formed, as in the bird, by a 

 proliferation of the epiblast. To 

 this mass of cells the hypoblast is 

 also partially adherent. At the front 

 end of the streak an epiblastic invo- 

 lution appears, which soon becomes 

 extended into a passage open at 

 both extremities, leading obliquely 

 forwards through the epiblast to 

 the space below the hypoblast. The 

 walls of the passage are formed of a 

 layer of columnar cells continuous 

 both with epiblast and hypoblast. 

 In front of the primitive streak the 

 body of the embryo becomes first 

 differentiated by the formation of a 

 medullary plate ; and at the same 

 time there grows out from the pri- 

 mitive streak a layer of mesoblast, 

 which spreads out in all directions between the epiblast and 



FIG. 126. SECTIONS THROUGH 

 AN EMBRYO OF LACERTA MURALIS 

 REPRESENTED IN FIG. HQ. 



m.g. medullary groove ; mep. 

 mesoblastic plate ; (p. epiblast ; hy. 

 hypoblast ; cK '. notochordal thick- 

 ening of hypoblast ; ch. notochord ; 

 tie. neurenteric canal (blastopore). 

 In E. ne points a diverticulum of 

 the neurenteric canal into the pri- 

 mitive streak. 



