A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMKS. 117 



Upon making an incision along one side of the sac to facilitate embedding, a 

 large cavity i.s found into which projects the embryonic anlage attached to the side 

 showing the free villi. Regarding the embryo 

 proper nothing more than a small, whitish, glob- 

 ular mass (yolk-sac) can be made out for fear 

 of injuring the embryonic structures. At this 

 time there were seen a few minute but distinct 

 strands traversing the cavity (exocoelom) and 

 connecting the inner surface of the vesicle with 

 the yolk-sac. Traction upon the margins of the 

 opening in the vesicle could be seen to have 



,. . , n FIG. 1. Schematic representation of embryo 



a very distinct enect upon the yolk-sac on ac- an d vesicle, x about 4.5. From photo- 

 count of the attachment of the above-mentioned SS^twfiAi .^of uc 

 filaments. 



II. THE EMBRYO AND ADNEXA. 



The main features of the embryonic anlage are shown in the text-figures 1, 2, 

 and 3 and in the photographs of the model (plate 4, figs. 1 and 2). The general 

 shape of the blastoderm is not unlike that of the Frassi embryo, but narrower and 

 as a whole very much larger. Its dimensions, determined on the reconstructions 

 (X 100), are 2 mm. in extreme length by about 75 mm. in breadth at the widest 

 point. The ventral surface of the embryonic disk presents a very slight ventral 

 concavity in the sagittal plane, while at right angles to this the same surface is for 

 the most part convex, i. e., projecting slightly into the yolk-sac. The dorsal surface 

 is in general more strongly convex, owing to the presence of the prominent ecto- 

 dermic folds. The amnion above completes roughly the curvature of the yolk-sac 

 below. The anterior extremity of the blastoderm is quite regularly rounded and, 

 especially on the left side, is undermined by shallow extensions of the exoco?lom; 

 the posterior half tapers evenly to a point. 



As may be seen in the dorsal view of the model, the appearance of the posterior 

 third of the embryonic disk is quite different from that of the middle and anterior 

 thirds. This posterior part contains the cloacal membrane and about the caudal 

 half of the primitive streak and is the most regularly formed part of the entire 

 blastoderm. The dorsal surface presents here, on either side of the median line, 

 two rather steep, even slopes (plate 3, fig. 1), the left slightly more extensive, which 

 extend from the region of the cloacal membrane and primitive streak to the attach- 

 ments of the amnion laterally. The primitive groove appears in the model simply 

 as the central, deepest portion of this valley-like area. The ventral surface of the 

 region in question is strongly convex from side to side. Distinct primitive folds can 

 not be made out. 



The central third of the embryo includes the anterior half of primitive streak 

 and the head-process region in front of it. Most conspicuous here are the two large 

 folds of ectoderm which extend from near the middle line to the attached border of 

 the amnion; the fold on the left side is broader and more regular than that on the 



