A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



417 



the embryonic plate. In the third stage the villi are branched and the size of the 

 chorion has increased relative to that of the embryo. The first three stages as a 

 group include all those specimens in which the primitive groove has not yet formed. 

 The fourth stage is marked by the presence of a primitive groove; there is also a 

 well-developed allantoic duct, the yolk-sac has become larger than the amniotic 

 vesicle, and the first steps in the formation of blood-vessels can be recognized in 

 the wall of the yolk-sac and in the chorion. In the fifth stage, in addition to a 

 primitive groove, there is a distinct head-process with its contained canal, and also a 

 completion-plate. Medullary folds can be recognized, and at the caudal end of the 

 primitive groove the ectoderm and endoderm unite in the formation of a cloacal 

 membrane. In the sixth stage there is further differentiation of the neurenteric 

 canal, formation of the chordal plate, beginning constriction of the gut-endoderm 

 from the remainder of the yolk-sac, and finally evidences of angiogenesis within 

 the body of the embryo. The fifth and sixth stages make up our third group of 

 specimens of the presomite period. 



GROUP 



A 



GROUP 



GROUPS 



A 



CHART 1. Size of the human ovum during the presomite period, showing range of the greatest 

 internal diameter of the chorion (in millimeters) of all the human embryos thus far pub- 

 lished, from the youngest-known stage up to the appearance of the somites. 



In addition to these morphological changes there is a corresponding change 

 in the size of the different ova. This is shown in chart 1, in which the size of the 

 ovum is represented by its most easily determined dimension the largest diameter 

 of the chorionic cavity. The range of this dimension, as can be seen in table 1 and 

 in chart 1, falls within limits that correspond consistently to the different stages 

 of development. In the only specimen in which there is a departure from this 

 dimensional curve (Strahl-Beneke ovum) the discrepancy is due to an extreme 

 elongation of the chorionic sac. 



From such morphological and dimensional criteria it is possible to determine 

 the relative development of any two specimens with a considerable degree of 

 accuracy, which is of the greatest importance in arriving at the true age of an 



