EARLY DEVELOPMENT OF THE LACERTILIA. 649 



the fusion of the epiblast and hypoblast with a number of cells 

 between them, which are probably derived from the lower rows 

 of the segmentation cells. At the front end of the streak a 

 passage arises, open at both extremities, leading obliquely for- 

 wards 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 primitive streak a layer 

 of mesoblast, which spreads out in all directions between the 

 epiblast and hypoblast. In the axis of the embryo the meso- 

 blast 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 becomes very soon 

 quite separated from the hypoblast, except at the hind end, 

 where it is continued into the front wall of the neurenteric pas- 

 sage. It is interesting to notice the remarkable relation of the 

 notochord to the walls of the neurenteric passage. More or less 

 similar relations are also well marked in the case of the goose 

 and the fowl (Gasser) 1 , and support the conclusion deducible 

 from the lower forms of vertebrata, that the notochord is essenti- 

 ally hypoblastic. 



The passage at the front end of the primitive streak forms the 

 posterior boundary of the medullary plate, though the medullary 

 groove is not at first continued back to it. The anterior wall of 

 this passage connects together the medullary plate and the noto- 

 chordal ridge of the hypoblast. In the succeeding stages the 

 medullary groove becomes continued back to the opening of the 

 passage, which then becomes enclosed in the medullary folds, 

 and forms a true neurenteric passage. It becomes narrowed as 

 the medullary folds finally unite to form the medullary canal, 

 and eventually disappears. 



1 Gasser, Der Primitivstreifen bd Vogelembryonen, Marburg, 1878. 

 B. 4 2 



