(',4 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



short portion of the dorsum of the embryo which is very nearly straight, while 

 beyond this is a gentle rounded elevation completing the embryonic body. Seen in 

 profile, No. 1878 is strikingly like the older but slightly shorter embryo (H 98), 

 1.27 mm., recently described by Wilson (1914, plate III). The Kromer-Pfannenstiel 

 embryo, while only a little longer (1.8 mm.) is much more advanced. The model 

 shows the body of this embryo to be practically straight (Kromer, 1903; Keibel and 

 Elze, 1908). Indeed, in the model there seems to be hardly room enough for the 

 heart and pericardial cavity, as they have been described and figured in section, 

 between the brain and the yolk-sac. 



There can be no doubt that the early development of the heart, and especially 

 of the myoepicardial mantle and pericardial ccelom, plays a very important role 

 in the lifting up of the head. This general effect on the anterior end of the embryo 

 could be accentuated only by the precocious development of the forebrain. Further 

 evidence of such an elevating process may be seen in the relation of the amniotic 

 reflection anteriorly, where, immediately below the stomodeum it is lifted up as a 

 saddle-like ridge by the large pericardial coelom beneath. In addition, one may 

 conclude that a large j^olk-sac would have a similar tendency to raise the heart and 

 with it, of course, the entire anterior end of the embryo. This might help explain 

 the condition in the Kromer embryo, where the yolk-sac is not very large. While 

 doubtless subject to not a little individual and relative variation, the yolk-sac, 

 throughout most of its history, especially later and possibly also very early, is 

 rather small as compared with the embryo and body-stalk. For a very brief 

 period, however, it undergoes a rapid increase in dimensions and the relative size 

 of the embryo and sac is, for a short time, reversed. In the Kromer specimen the 

 relation of the two is being changed for the second and last time. Certainly the 

 embryonic body is weaker and more pliable about its middle than anj^where else, 

 and all things considered it is difficult to see how it could possibly be bent in the 

 opposite direct ion. A dorsal flexure, normal or otherwise, is what one might expect 

 at this time and, until more is definitely known as to the possible factors involved, 

 some degree at least of such a flexion will have to be looked upon as normal. On 

 the whole, it would be better to speak of an elevation or erection of the head, as also 

 suggested by Wilson, and so put the emphasis where it undoubtedly belongs. There 

 is no reason why a curve of this character may not later be wiped out entirely, as 

 happens to so many others in the embryo, of which we will mention only the dorsal 

 curvature of later stages, which is subject to very considerable individual variation. 

 The deep, sharp kinks shown by the His embryos Lg. and Sch., 2.15 and 2.2 mm. 

 respectively, may very well lie abnormal, and, except their direction, they appar- 

 ently have little if anything in common with the so-called flexure which we have 

 just discussed. In the same category would fall the embryo Delaf. of Eternod 

 (1909), noted as "2.1 mm. (?) enselle," admittedly an old alcohol specimen. These 

 kinks involve the body at a definitely more caudal point. They occur at a later 

 stage in development, at a time when the diminishing yolk-sac not only offers less 

 support to the embryo, but would be in a favorable position, on account of its 

 narrowing attachments, to actually exert some traction upon the delicate body of 



