402 



A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



syncytial clumps resembles a process of excavation, and in this sense the syncytial 

 clumps would have to be regarded as degenerate trophoblast. In text-figure 4 

 several varieties of syncytial masses (marked /) can be seen in different degrees of 

 formation and in the process of detachment from the adjoining trophoderm. The 

 picture presented by these masses is very suggestive of their retrograde character. 

 The vascularization of the chorion will be described in connection with that 

 of the embryo and of the yolk-sac. 



Decidua 



Chorionic 

 Mesoderm 



Chorionic 

 [ Ectoderm 



(jSyncytial laye 

 [Cellular layei 



FIG. 4. Detailed structure of chorionic membrane and villi, showing the transition into trophoMast and decidua. Syneytia 

 masses in different stages of formation are marked "f." Section 168, enlarged 125 diameters. 



EMBRYO. 



The form and relation of the embryo and adnexa are shown in plate 1, figures 

 5 and 6, and typical sections through this region are reproduced in figures 9 to 16, 

 plates 2, 3 and 4, which will be repeatedly referred to. For convenience of descrip- 

 tion these structures will be taken up separately as the embryonic plate, amnion, 

 yolk-sac, and body-stalk. 



EMBRYONIC PLATE. 



This is somewhat oval in outline. Caudally, it narrows rapidly and is some- 

 what pointed where it terminates at the body-stalk. In its widest diameter it 

 measures 0.75 mm.; it is 1 mm. long in its median axis. The sections cut it trans- 

 versely only in the caudal portion; for the greater part of the plate they meet it 

 tangentially or in a very oblique direction. For its form we must depend prin- 

 cipally upon the reconstruction. From this it can be seen that there is a distinct 

 primitive groove in its caudal fourth, measuring 0.25 mm.; otherwise the plate is 

 smooth, with a slight tendency toward the formation of bilateral, low marginal 

 ridges. There is, however, no evidence of a medullary groove. In the more trans- 



