FORMATION OF THE ALLANTOIS. 



45 



Like the amnion. an allantois is only formed in the embryo of reptiles, birds, and mammals. 1 

 It varies chiefly in the extent to which its hypoblastic part becomes developed. In reptiles. 



Fig. 47. LONGITUDINAL SECTION AT A SLIGHTLY LATER STAGE THAN THAT SHOWN IN FIG. 46. 



The allantoic protrusion now springs from the ventral wall of the hind gut. Lettering as in fig. 46. 

 (FromKolliker.) 



Fig. 48. EARLY HUMAN EMBRYO. (From His after Coste.) 



The embryo is enclosed within the amnion am, and is attached at its caudal end by the allantoic 

 stalk, all, to the chorion. The yolk sac, ys, is still distinct from the allantoic stalk. 



Fig. 49. DIAGRAM OP LONGITUDINAL SECTIONS THROUGH THE HUMAN OVUM AT SUCCESSIVE STAGES 



SHOWING THE DIPPING DOWN OF THE EMBRYO-RUDIMENT INTO THE BLASTODERMIC VESICLE, AND 

 THE FORMATION OP THE FOREGUT, AMNION, AND STALK OF THE ALLANTOIS BY THE FOLDING OF 

 THE BLASTODERM. (HlS.) 



Am, head fold of the amnion (pro-amnion in b and c) ; Kb, yolk sac. a and d are conditions of the 

 embryo which have been seen and described ; b and c are intended to show how the conditions found 

 in d may be brought about, and especially how the stalk of the allantois may be regarded as a direct 

 continuation of the posterior end of the embryo, which according to His does not lose at any time its 

 connection with the chorion or villous external membrane of the ovum. The curved dotted lines in c 

 indicate the formation of the amnion and false amnion by the upgrowing of lateral folds (which have 

 not as yet met in the median line). 



1 For a discussion of the origin and meaning of the amnion and allantois, see Balfour, " Comparative 

 Embryology," II., p. 256, 



