COMPARISON WITH RABBIT EMBRYOS. 483 



Another difference of even greater importance between the 

 two embryos is found in the relations of the yolk-sac in the 

 two cases respectively, to which attention has already been 

 directed. In the rabbit, the yolk-sac (Fig. 146, ys) is part of 

 the blastodermic vesicle itself; while in the human embryo (cf. 

 Fig. 188) it lies freely within this. This difference is explained 

 by Keibel as due to a relatively early extension of the splitting 

 of the mesoblast, in the human embryo, right round the lower 

 half of the blastodermic vesicle. In the rabbit (Fig. 147) the 

 mesoblast, and consequently the cavity, C, between its layers, only 

 extends halfway round the blastodermic vesicle, stopping at 

 the sinus terminalis, ST. If, in the rabbit, the mesoblast, and 

 the split between its somatic and splanchnic layers, were to 

 extend round to the lower pole of the blastodermic vesicle, then 

 the yolk-sac would be completely split away from the wall of the 

 vesicle, and a condition similar to that of the human embryo 

 would be attained. 



No stages intermediate between Eeichert's ovum and His' 

 embryo E, or V. Spee's embryo, have yet been described. The 

 gap, though probably only a slight one in actual time, is of 

 great importance ; for, while Eeichert's ovum has no trace of 

 an embryo, His' embryo E possesses neural groove and folds, 

 heart and yolk-sac, and has both amnion and allantois well 

 developed. 



His has attempted to bridge over the interval, and has 

 given a series of diagrams, reproduced in Figs. 186-188, show- 

 ing hypothetical intermediate stages. 



The figures represent diagrammatic longitudinal sections 

 through embryos at successive stages of development, and should 

 be compared with Fig. 174, which represents a similar section 

 through Eeichert's ovum. 



In Fig. 186, which is a hypothetical stage, the commence- 

 ment of the formation of the embryo is indicated. The 

 embryonal or germinal area has become somewhat depressed, 

 but at its anterior end, to the right in the figure, is lifted up 

 by the commencing head fold. 



In Fig. 187, also a hypothetical stage, the general depression 

 of the embryonal area has increased, the embryo being pushed 

 down into the blastodermic vesicle. The head fold has deepened, 



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