244 



EMBRYOLOGY. 



fundament of the embryo itself. Either the ova were already more 

 or less pathologically altered, or the contents were considerably 

 damaged in consequence of the method of preservation and by the 

 preparation. At all events with other investigators one, I think, 

 may conclude from the condition of the chorion that the embryo 

 must have been in an advanced stage, in which germ-layers, yolk- 

 sac, and amnion were already formed. 



This assumption is all the more reasonable, since well-developed 

 embryos from blastodermic '.vesicles which were only a few milli- 

 metres larger have been described by COSTE, ALLEN THOMSON, His, 

 and others. In these cases the head-end of the embryo only is 

 rather sharply differentiated from the yolk-sac, which is continuous 

 with the fundament of the intestine throughout nearly its entire 



B 



Fig. 140. The human ovum at an early stage of development. 



A and B, Front and side views of a human ovum of 12 to 13 days, figured by REICHERT. e, The 



part designated by REICHERT as embryonic spot. From QUAIN'S " Anatomy." 

 C, An ovum of 4 to 5 weeks, showing the general character of the villous membrane before the 



formation of the placenta. A part of the wall of the ovum is removed in order to show the 



embryo in situ. After ALLEN THOMSON, from KULLIKER'S " Entwicklungsgeschichte des 



Menschen, etc." 



length. The neural canal is not yet closed, but the amnion never- 

 theless is completely developed, and in fact lies almost in contact 

 with the embryonal body ; at its posterior end it is connected with 

 the chorion by means of a short cord, which is connected with the 

 fundament of the allantois and has been named the belly-stalk 

 (Bauchstiel) by His. 



Also in the only slightly older embryo of COSTE (fig. 141) in which 

 the neural tube is closed, the body distinctly segmented (us), the 

 head provided with visceral arches (vb), behind the latter the 

 heart (h) recognisable, and the yolk-sac (ds) further constricted off- 

 a short belly-stalk (bst) is present. It is composed of the amnion 

 (am 1 ) drawn out to a point and of a connective-tissue cord, which 

 arises from the ventral surface of the embryo out of the intestinal 

 cavity of the pelvic region, encloses at its attached end a small cavity 



