134 



THE HUMAN EMBRYO. 



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occupies a longitudinal median position. In this embryo it has already become 

 a relatively large organ and the tube itself is strongly bent. No anlage of the 



eye or ear was distinguished. The amnion was a thin, 

 transparent membrane enveloping the embryo quite 

 closely. The closeness of the amnion to the embryo 

 was probably accidental (compare Figs. 69 and 71). 

 The chorion was covered externally by branching villi ; 

 its diameter, including the villi, was 18 mm. 



Another embryo, the position of which in the series 

 of known stages has long been a matter of dispute, I 

 feel, after renewed study, must be assigned to a place 

 very close to KoUmann's embryo just described. The 

 specimen in question was figured by Coste in his 

 monumental "Atlas of Embryology."* The embryo 

 was enclosed in a villous chorion (Fig. 69) and was pro- 

 vided with a large vitelline sac, Vi, having a very broad 

 connection with the embryo and covered with a network 

 of vessels, in which was a fluid not yet red. A thick 

 body-stalk, Al, can be seen running from the under side 

 of the embryo's tail to the chorion; from the anterior 

 side of the stalk springs the amnion. Am, completely 

 inclosing the embryo. It is important to notice that in 

 this, as in still older embryos, the disposition of the 

 amnion is essentially the same as in the earliest stages ; 

 the line of attachment of the amnion is down the sides 

 of the allantois and around the embryo about on a line 

 with the top of the yolk. As regards the embryo, it is 

 drawn slightly canted on to its left side; its back is 

 concave ; the head end is thickest ; behind and below it 

 can be seen the heart, already a bent tube, shining* 

 through; and on the dorsal side, the light-looking 

 oesophagus is distinguishable; in the figure a wedge-shaped shadow intervenes 

 between the straight oesophagus and the bent heart ; the heart causes a conspicu- 



FiG. 70. — Embryo of Fig. 69, 

 Separated from the 

 Yolk-sac and Viewed 

 from the Under Side. 



Am, Amnion. Ht, Heart. Spl, 

 Splanchnopleure extending 

 beyond the embryo to form 

 the yolk-sac. S, Noto- 

 chord with a row of primi- 

 tive segments on each side. 

 Al, ^ody-stalk. 



* The greatest difficulty comes from Coste's statement as to the magnification of his drawings, according to 

 which the embryo must have been about 4.4 mm. long, or nearly double the length which we now know to be 

 normal for embryos in the stage in which this one seems to be. Other difficulties arise because Coste has given 

 no further description of this embryo than that which appears in the explanation of his plate. Neither that 

 explanation nor the figures themselves afford any information concerning the dorsal side of the embryo or as to 

 whether it had a partially open medullary groove or not. Coste's figures indicate that thirteen or fourteen seg- 



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