1( 



FORMATION OF THE LAYERS. 



mep 



niep 



of the blastoderm. The embryonic shield is mainly distinguished 

 from the remainder of the blastoderm by the more columnar character 

 of its constituent epiblast cells. It is somewhat pyriform in shape, 

 the narrower end corresponding with the future posterior end of the 

 embryo. At the hind end of the shield a somewhat triangular 

 primitive streak is formed, consisting of epiblast continuous 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 uf cells 

 the hypoblast. is also partially adherent. At the front end of the 

 streak an epiblastic involution 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 hy- 

 poblast. 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 primitive 

 streak a layer of mesoblast, which spreads 

 out in all directions between the epi- 

 blast and hypoblast. In the region of the 

 embryo the mesoblast plate is stated by 

 Kupffer and Benecke to be continuous 

 across the middle line, but this appears 

 very improbable. In a slightly later 

 stage the medullary plate becomes 

 marked by a shallow groove, and the 

 mesoblast of the embryo is then un- 

 doubtedly constituted of two lateral 

 plates, one on each side of the median 

 Line. In the median line the notochord 

 arises as a ridge-like thickening of the 

 hypoblast, which is continued posteriorly 

 into the front wall of the passage men- 

 tioned above. 



The notochord does not long remain 

 attached to the hypoblast, and the 

 separation between the two is already 

 effected for the greater part of the 

 length of the embryo by the stage re- 

 presented in fig. 129. Fig. 126 repre- 

 sents a series of sections through this 

 embryo. 



In a section (A) through the trunk 

 of the embryo a short way in front of 

 the primitive streak, there is a medullary 



Fig. 128. Sections THROtroH 



AN EMBRYO- OF LaCERTA MURALIS 

 REPRESENTED IN FIG. 129.. 



m.g. medullary groove ; mep. 

 mesoblastic plate; ep. epiblast; 

 hy. hypoblast; ch'. notochordal 

 thickening of hypoblast ; ch. noto- 

 chord; ne. neureiiteric canal (blas- 

 topore). In E. nc points a diver- 

 ticulum of the neurenteric canal 

 into the primitive streak. 



