412 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



as seen in their figures 41 and 42. More certain than either of the last two phe- 

 nomena is the beginning appearance of the primitive streak. In their figures 25 

 to 34 it can be seen that the ectoderm of the caudal end of the embryonic plate 

 fuses with the mesoderm, producing the appearance of a primitive streak, although 

 there is no distinct groove. In this ovum, as in the Herzog specimen, the yolk- 

 vesicle is distinctly larger than the amniotic vesicle, differing in this respect from 

 all those previously mentioned. The Herzog and Strahl-Beneke specimens are on 

 the border-land between groups 1 and 2, and, except for the absence of a primitive 

 groove, could be grouped with the "v. H" ovum of Graf v. Spee. 



Together with the Strahl-Beneke specimen should be mentioned the Merttens 

 (1894) ovum, which is about the same size. Unfortunately, the sections are incom- 

 plete, being fragments from curettage, so that it is not possible to determine much 

 regarding the structure of the embryo. The chorion, however, is well preserved 

 and acquires great importance because of the valuable clinical history that accom- 

 panies the specimen. Another of about the same age as the Strahl-Beneke ovum 

 is No. 763, of the Carnegie Collection, which has been mentioned by Mall (1915, 

 p. 22), and a photograph of which is shown in our figure 7, plate 1. The specimen 

 was found in curettage material and the series is therefore incomplete. The tissue, 

 however, is in an excellent state of preservation and the structure of the chorionic 

 membrane and villi is very well shown, as can be seen in the photograph. The 

 internal diameters of the chorion in the largest section are 2.5 by 1.2 mm., and the 

 average length of the villi is 0.5 mm. 



GROUP 2. EMBRYOS IN WHICH THE PRIMITIVE GROOVE is PRESENT. 



The embryos belonging in this group have a primitive groove but no medullary 

 groove or neurenteric canal. Five such specimens are referred to in the literature, 

 and in this group the Mateer embryo must be placed. They are all much alike in 

 size and form and evidently there is very little difference in their degree of develop- 

 ment. As a group they are of a size intermediate between groups 1 and 3. 



The well-known "v. H" ovum of Graf v. Spee (1896) is perhaps a little less 

 developed than the others. In the relatively small size and spherical form of its 

 amniotic vesicle it resembles some of the older specimens of group 1 and its embry- 

 onic shield is quite like that of the Jung specimen except for the presence of a primi- 

 tive groove. It has, however, a larger chorion (average internal diameter 4 mm.) 

 with freely branching villi. The yolk-sac is much larger than the amniotic cavity, 

 and there is a well-developed allantoic duct extending from it into the body-stalk. 

 On its ventral pole are found numerous blood-islands. In all these respects it con- 

 forms to the other specimens of group 2. 



Lewis (1912) has pictured and briefly described the Minot embryo (Harvard 

 series, No. 825), and I am informed that a more complete description of it is now 

 in course of preparation. This embryo resembles very closely the "v. H" embryo 

 of v. Spee, both in size and in form. It differs in that the amniotic vesicle is more 

 flattened and the embryonic shield correspondingly larger. Whether the spherical 

 form of the amniotic vesicle normally precedes the flattened form usually met with 



