412 JAS. P. HILL. 



the yolk splaiiclmopleure. The allantoic stalk with its three 

 vessels {all. a. and all. v.) is seen to emerge from under the 

 embryo's right side, and at its distal end the vessels spread out 

 on what is the inner wall of the vesicular portion of the 

 allantois. The distribution and mode of branching of the 

 allantoic vessels is clearly shown. As already described, the 

 vein [all. v.) is formed by the union of two factors which 

 accompany the corresponding arteries [all. a.). 



The placental area is discoid in shape and^ from the presence 

 of a prominent rim on the side visible, appears somewhat 

 depressed below the general surface of the uterus. In sections 

 it is found to commence a little behind the anterior margin of 

 the flexed head end of the embryo, and to terminate some 

 distance behind the level of its curved posterior end. Its 

 length, in the direction of the long axis of the embryo, is thus 

 about 9 mm., while in its middle region, following the folds, 

 it has a breadth of about 12 mm. transversely to the long axis 

 of the embryo. 



The flattened allantoic stalk has an approximate length 

 of 8 mm. 



The course of the vitelline vessels is also shown in the figure. 

 Leaving the yolk-stalk, the artery [vit. a.) passes obliquely 

 backwards, traversing the yolk splanchnopleure [y. spl.) to 

 reach the vascular omphalopleure [vase, omph.), and there it 

 almost immediately divides into two right and left branches, 

 which form the sinus terminalis [s. t.), a portion of which is 

 visible. The area outside the sinus (below) in the figure is 

 part of the bilaminar omphalopleure [bil. omph.). From the 

 yolk-stalk the two factors [vit. v.) of the vitelline vein pass 

 anteriorly in the yolk splanchnopleure, and gradually diverging 

 from each other, they pass over into the vascular omphalo- 

 pleure, where they are formed by the union of lesser factors 

 coming from the capillary system of the vascular area. 



I. Uterus. — The serosa, muscularis, and corium are es- 

 sentially the same as in Stage C. 



[a) Allantoic Placental Syncytium. — This presents 

 the same general features as the corresponding area in Stage C. 



