128 WALTEE HEAPE. 



the primitive streak into a short anterior portion which projects 

 upwards along the floor of the medullary groove at its hind 

 end, and into a larger posterior portion which forms the wall 

 of the allantois (figs. 33 and 35, Stage h, and figs. 48 and 50, 

 Stage j). 



The dorsal pit I have mentioned gives rise to the anus ; this 

 structure is therefore formed in the middle of the primitive 

 streak in the Mole, in the same position as Weldon (No. 11) 

 pointed out the anus of Lacertilia is formed. 



The Amnion. — The amnion is first formed, as I have before 

 described (p. 109), at the hind end of the embryo ; extending 

 thence forwards, and being met by the lateral folds of the 

 amnion which also grow, in the first place, from behind forwards 

 (figs. 26, 28, 35, and 50). This portion of the amnion is formed 

 as in the Chick of a double fold of somatopleure. Immediately 

 upon the junction of the two lateral folds and the formation 

 of true and false amnion, the epiblast of the false amnion, 

 which is shown in fig. 28, unites eventually with the neigh- 

 bouring uterine tissue, and the thin sheet of somatic mesoblast 

 alone remains between the uterine wall externally and the 

 splanchnic mesoblast within. 



At the front end of the embryo a different structure is found 

 to exist. The lateral folds in this region are similar to the 

 posterior portion of these folds, but the median anterior fold 

 of the amnion is different inasmuch as it is formed solely of 

 epiblast and hypoblast (fig. 34). Although the amnion at the 

 anterior end is not formed until some considerable time after 

 the mesoblast of the embryo has extended to the front end of 

 the embryonic area, and although this mesoblast has ex- 

 tended laterally over the vesicle throughout the whole length 

 of the embryonic area, it only extends forwards for a very 

 short distance, and does not grow between that portion of the 

 epiblast and hypoblast which gives rise to the anterior fold of 

 the amnion. Consequently, when the head of the embryo 

 becomes projected anteriorly over the yolk-sac, as it does 

 first in Stage g (fig. 6), and then bends downwards, forming 

 for itself a pit on the surface of the yolk-sac, the walls of this 



