72 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



The segments thus far differentiated lie just in advance of the center of the 

 dorsal concavity and a short distance farther forward the unsegmented mesoderm 

 becomes continuous with the walls of the pericardial cavity. As regards the nervous 

 system, their location would be about the junction of brain and cord and we may 

 look upon them as occipital somites. In the cervical somites which will form later, 

 one may expect a better differentiation and sharper demarcation both between the 

 individual somites and between these and the lateral mesoderm. Aside from the 

 pericardial cavity (described later) and the myoccels, practically the only evidence 

 of the formation of the intraembryonic ccelom is found in the presence of a number 

 of small, tubular prolongations of the exoccelom into the lateral mesoderm. They 

 present very much the same appearance as shown by Dandy (1910), but are rather 

 smaller. A few other unconnected spaces occur in the lateral mesoderm, forerun- 

 ners of the embryonic body-cavity. 



VASCULAR SYSTEM. 



By far the most interesting and important results of the study of this embryo 

 are those which pertain to early phases of vascular development. For the sake of 

 convenience in treatment we may recognize four regions, viz., the chorion, body- 

 stalk, yolk-sac, and embryo, which are also the various regions in which independent 

 blood-vessel formation has occurred, or where this process is still more or less active. 

 As will be seen later, these vascular areas are not only almost entirely independent 

 of each other, but the primitive vessels within these various regions show certain 

 differentiating characters. 



CHORION. 



Blood-vessels are present in large numbers throughout that portion of the 

 vesicle wall which forms a part of the series with the embryo (fig. 3) . The remainder 

 of the chorion is also doubtless vascularized, but probably to a somewhat less 

 extent. In the villi vessels are abundant as slender anastomosing channels or cords, 

 continuous with the deeper vessels of the chorionic wall, presenting the picture of a 

 loose, wide-meshed plexus. While for the most part forming a continuous network, 

 there are certainly many detached strands, although, in view of the beginning 

 abnormal cytological changes, which are more marked here than anywhere else in 

 the specimen, this condition need not be looked upon as representing the usual 

 method of development. The general swelling of the stroma in many of the villi 

 may be in some measure responsible for the presence of what appear to be solid 

 cords of cells rather than open vessels, although the latter do occur, often quite 

 small but not infrequently of considerable size, particularly near the bases of the 

 larger villi. 



In the chorion proper, vascular channels are much more numerous and well 

 defined, while the stroma is in a practically normal condition. The vascular picture 

 here is in marked contrast with that in the villi, or indeed with that of all other 

 parts of the ovum. There exists a rich plexus of vessels occupying the entire thick- 

 ness of vesicle wall. They have in general an arrangement parallel with the surface 

 and vary considerably in caliber. In contradistinction to the vessels in the villi 



