REPTILIA, 



171 



to be formed as usual of hypoblast (hy] enveloped by splanchnic mesoblast 

 (me), and projects into the section of the body cavity present behind the 

 embryo. Its position in the body cavity is the cause of its somewhat 

 peculiar appearance in the figure. Had the whole section been, represented 

 the allantois would have been enclosed in a space between the serous mem- 

 brane (se) and a layer of splanchnic mesoblast below which has also been 

 omitted in fig. B '. It still points directly backwards, as it primitively does 

 in the chick, vide fig. 123 A, and Gasser, No. 127, PI. v. figs. 1 and 2. I do 

 not understand the apparently double character of the lumen of the allantois. 

 In the next section (not figured) the lumen of the allantoic stalk is larger, 

 but still apparently double, while in the last section (D) the lumen is 

 considerably enlarged and single. The neurenteric canal appears to close 

 shortly after the stage last described, though its further history has not 

 been followed in detail. 



A 



aid 



FIG. 1'28. FOUR TRANSVERSE SECTIONS THROUGH THE HINDER END OF A YOUNG 



EMBRYO OF LiACERTA MUBALIS. 



Sections A and B pass through the whole embryo, while C aud D only pass through 

 the allantois, which at this stage projects backwards into the section of the body cavity 

 behind the primitive streak. 



ne. neurenteric canal; pr. primitive streak; hg. hindgut ; liy. hypoblast; pp. body 

 cavity ; am. amuiou ; sc. serous envelope (outer limb of the amniou fold not yet 

 separated from the inner limb or true arnnion); aJ. allantois; me. mesoblastic wall of 

 the allantois ; v. vessels passing to the allautois. 



1 Owing to the difficulty of procuring material I have only been able to prepare the 

 two sets of sections just described, and in the absence of a fuller series there are some 

 points in the interpretation of the sections which must remain doubtful. 



