PLATE 1. 



FIG. 5. Frontal view of wax-plate reconstruction of Mateer embryo, showing yolk-sac and embryonic plate outlined 

 by the cut edge of the amnion, which is shown as removed. The stump of the body-stalk can be seen with 

 the allantois at its center. Extending forward from the caudal end of the embryonic plate is a well-defined 

 primitive groove. Enlarged 60 diameters. 



FIG. 6. Median view of wax-plate reconstruction of Mateer embryo showing the form of the amn'otic cavity and its 

 relation to the chorionic membrane and the yolk-sac. The allantois projects through the body-stalk and is 

 interrupted near its center. In the loose tissue caudal to the body-stalk is an epithelial mass constituting 

 the amniotic vesicle of the twin embryo. Enlarged 60 diameters. 



FIG. 7. Photograph of section through embryo No. 763, Carnegie Collection. This specimen would belong among 

 those in our group 1 having branched villi. The embryo, consisting of two vesicles, can be seen in the center. 

 The series being incomplete, the definite form of the embryo can not be made out. This specimen is dis- 

 tinctly younger than the Mateer specimen and its villi are to be compared with those in figure 8. Enlarged 

 30 diameters. 



FIG. 8. Photograph of a typical larger villus of the Mateer specimen, showing transition into trophoblast. For a 

 detail of these structures see text-figure 4. Enlarged 50 diameters. 



