2-24 HUM AX OVUM. 



The embryo at this stage (fig. 166), which is about equivalent to that of a 

 chick 011 the fourth day, resembles in almost every respect the normal 

 embryos of the Amniota. The cranial flexure is as pronounced as usual, 

 and the cerebral rtgioii has now fully the normal size. The whole body 

 soon becomes flexed veiitrally, and also somewhat spirally. The yolk- 

 sack (b) forms a small spherical appendage with a long wide stalk, and the 

 embryo (B) is attached by an allantoic stalk with a slight swelling (all), 

 probably indicating the presence of a small hypoblastic diverticulum, to 

 the inner face of the chorion. 



A remarkable exception to the embryos generally observed is afforded 

 by an embryo which has been described by Krause (No. 235). In this 

 embryo, which probably belongs to the third week of pregnancy, the limbs 

 were just commencing to be indicated, and the embryo was completely 

 covered by an am n ion, but instead of being attached to the chorion by an 

 allantoic cord, it was qtiite free, and was provided with a small spherical 

 sack-like allantois, very similar to that of a foui'th-day chick, projected 

 from its hind end. 



FlG. 107. FlGUEES SHEWING THE EARLY CHANGES IN THE FOEM OF THE HUMAN 



HEAD. (From Quain's Anatomy.) 



A. Head of an embryo of about four weeks. (After Allen Thomson.) 



B. Head of an embryo of about six weeks. (After Ecker.) 



C. Head of an embryo of about nine weeks. 



1. mandibular arch ; 1'. persistent part of hyomandibular cleft; a. auditory vesicle. 



No details are given as to the structure of the chorion or the presence 

 of villi upon it. The presence of such an allantois at this stage in a 

 human embryo is so tmlike what is usually found that Krause's statements 

 have been received with considerable scepticism. His even holds that the 

 embryo is a chick embryo, and not a human one; while Kiilliker regards 

 Krause's allantois as a pathological structure. The significance to be 

 attached to this embryo is dealt with below. 



A detailed history of the further development of the human embryo 

 does not fall within the province of this work ; while the later changes in 

 the embryonic membranes have already been dealt with (pp. 202 205). 



For the changes which take place on the formation of the face I may 

 refer the reader to fig. 167. 



The most obscure point connected with the early history of the human 

 ovum concerns the first formation of the allantois, and the nature of the villi 



