A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 415 



their size, these ova represent a stage definitely in advance of that seen in the 

 specimens of group 2. 



In the Grosser specimen an amniotic duct is described as connecting the 

 amniotic ectoderm with the ectoderm of the chorionic membrane. A similar struc- 

 ture has been seen in the much younger Strahl-Beneke (1910) embryo. In the 

 Strahl (1916) specimen there exists in the body-stalk a small remnant of what seems 

 to be an amniotic duct in process of disappearing. In the Ingalls specimen, how- 

 ever, neither the presence of the amniotic duct nor evidence of its recent disappear- 

 ance could be made out. There is a short amniotic diverticulum extending from 

 the amniotic cavity towards the allantois, which is regarded by Ingalls (p. 21) as 

 comparable to the canalis amnioallantoideus connecting these cavities in certain 

 reptiles. 



The Bayer ovum described by Keibel (1890), judging from its size and general 

 form, would represent about the same stage of development as those just men- 

 tioned. Unfortunately, the preservation is not good, and much of the detailed 

 structure of the specimen is lost. The embryonic shield shows a well-developed 

 primitive streak, but whether a head-process and a neurenteric canal are present 

 could not be verified. 



The Frassi (1907) specimen, which was studied in Professor Keibel's labora- 

 tory, is a little larger than those thus far mentioned in this group. It possesses a 

 medullary groove bordered by evidences of medullary folds. A canalis neuren- 

 tericus connects the caudal end of the floor of the groove with the amniotic cavity. 

 A definite cloacal membrane is present. The amniotic and vitelline cavities are 

 distended and thin-walled ; in structure they are essentially the same as those found 

 in all of the specimens of groups 2 and 3. The same is true of the allantois, the 

 chorion and the chorionic villi. These features do not offer definite criteria for 

 determining the relative degree of development of the specimens of these two 

 groups, nor can the observations on blood-vessel formation be safely used for this 

 purpose, for the reason that the reported presence or absence of blood-islands arid 

 blood-vessels in the various parts depends largely upon the state of preservation 

 of the specimen, and also upon the observer's familiarity with the early form of 

 these structures. The Frassi specimen showed no evidence of an amniotic duct. 



The last two specimens that should be included in this group are the frequently 

 referred to Vuillet specimen of Eternod (1899) and the Glaevecke specimen of v. 

 Spec (1899). One should, perhaps, add to these the Triepel (1916) specimen, which 

 seems to be in about the same stage of development, although in not quite as good 

 condition. The v. Spee ovum is slightly more advanced, both in form and size, 

 than that described by Eternod ; otherwise there is a very close resemblance between 

 the two. Each has a distinct medullary groove, bordered by broad medullary folds 

 corresponding to the head-region. A well-defined chordal plate lies along the 

 ventral surface of the medullary groove and terminates caudally at a large, distinct 

 neurenteric canal, where its cells are continuous with those of the ectodermal cells 

 of the medullary plate. Caudal to the neurenteric canal, in the region of the primi- 

 tive streak, the axis of the embryo bends sharply ventralward to terminate in the 



