A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, WITH SPECIAL 

 REFERENCE TO THE VASCULAR SYSTEM. 



By N. William Ingalls. 



The embryo which forms the subject of this communication is No. 1878 of the 

 Carnegie Embryological Collection. Data relative to its age are rather indefinite, 

 but it is apparent that the menstrual age is in excess of the real or fertilization age. 

 Further evidence of this is found in the fact that the tissues of the ovum (and 

 this applies also to the endometrium) had begun to suffer slightly, with the result 

 that the fixation and staining reactions are not all that could be desired. These 

 changes are not, however, of such a nature or degree as to raise doubts about the 

 specimen being normal. They have been further complicated by what has been in 

 some respects an unfavorable plane of section, so that at times considerable diffi- 

 culty has been encountered in determining certain of the finer morphological details 

 presented by this particular stage. This account, therefore, will confine itself to a 

 brief description of the essential features of the embryonic body and of the struc- 

 tures immediately adjoining. We shall not enter upon any discussion of the findings 

 in the yolk-sac or chorion, or of the finer histological details of the embryo itself. 

 Evidences of mitotic activity are encountered, but they are not especially frequent. 



Concerning the gross specimen we may quote the laboratory notes as follows : 



"Main mass of specimen composed of decidua, clots and scrapings from curettage. 

 In addition to this there is an isolated flattened ovum measuring 7.5 X 10.5 X 1 2 mm. The 

 specimen is stained pink, like the scrapings obtained from curettage. The bare surface of 

 the chorionic vesicle, which comprises about one-third of area of ovum, is folded somewhat. 

 The villi seem to be well-developed, but are absent also on a narrow strip in the side oppo- 

 site the bare area. The bare areas look rather thick-walled and the villi rather clubbed. 



"Upon opening the ovum from the bare area it felt rather firm. The chorion was thick 

 and most of the interior was filled with a web-like magma, small portions of which were 

 not unlike absorbent cotton. It was quite easy, however, to remove all the magma without 

 injury to the embryo. The latter looked somewhat mottled, especially in the region of the 

 yolk-sac, which lay somewhat at right angles to the amnion. At. the point of union of the 

 two a somewhat elongated, opaque disc could be seen quite distinctly." 



EXTERNAL FORM, EMBRYO AND ADNEXA. 

 The form of the embryonic body (figs. 1, 2, 3, and 4) is in general quite similar 

 to a larger and older specimen, No. 391, 2 mm. (Dandy, 1910), and a much more 

 advanced one, No. 1201, also 2 mm., both of the Carnegie Collection. In all three, 

 as in so many other embryos of about this stage, there is a well-marked dorsal con- 

 cavity. In No. 1878 this flexure is very abrupt, even more marked than in No. 1201 

 (Evans and Bartelmez, 1917), and is most acute about, or slightly in advance of, the 

 fine between the future brain and cord. Posterior to the angle thus formed is a 



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