648 EARLY DEVELOPMENT OF THE LACERTILIA. 



or else the passage have been exceptionally short in this embryo, 

 since in an older embryo it could not all be seen in one section. 



The front wall of the passage is continuous with the noto- 

 chord, which for two sections or so in front remains attached to 

 the hypoblast (figs. 2 and 3). Behind the perforation in the floor 

 of the medullary groove is placed the primitive streak (fig. 5), 

 where all the layers become fused together, as in the earlier, 

 stage. Into this part a narrow diverticulum from the end of the 

 medullary groove is continued for a very short distance (vide 

 fig. 5, me.). 



The general features of the stage will best be understood by 

 an examination of the diagrammatic longitudinal section, repre- 

 sented in woodcut, fig. i. In front is shown the amnion (<?;;/.), 

 growing over the head of the embryo. The notochord (ch.} is 

 seen as an independent cord for the greater part of the length of 

 the embryo, but falls into the hypoblast shortly in front of the 

 neurenteric passage. The neurenteric passage is shown at ne., 

 and behind it is shown the primitive streak. 



In a still older stage, represented in surface view on PI. 29, 

 fig. C, the medullary folds have nearly met above, but have not yet 

 united. The features of the passage from the neural groove to 

 the hypoblast are precisely the same in the embryo just described, 

 although the lumen of the passage has become somewhat nar- 

 rower. There is still a short primitive streak behind the embryo. 



The neurenteric passage persists but a very short time after 

 the complete closure of the medullary canal. It is in no way 

 connected with the allantois, as conjectured by Kupffer and 

 Benecke, but the allantois is formed, as I have satisfied myself 

 by longitudinal sections of a later stage, in the manner already 

 described by Dobrynin, Gasser, and Kolliker for the bird and 

 mammal. 



The general results of Kupffer's and Benecke's observations, 

 with the modifications introduced by my own observations, are 

 as follows : After the segmentation and the formation of the 

 embryonic shield (area pellucida) the blastoderm becomes dis- 

 tinctly divided into epiblast and hypoblast 1 . At the hind end of 

 the shield a somewhat triangular primitive streak is formed by 



1 This appears to me to take place before the formation of the embryonic shield. 



