MAMMALIA. 269 



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 amnion, but instead of being attached to the chorion by an 

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

 sack-like allantois, very similar to that of a fourth-day chick, projected from 

 its hind end. 



FIG. 167. FIGURES SHEWING THE EARLY CHANGES IN THE FORM 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. 



i. mandibular arch; i'. 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 unlike 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 Kolliker 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. 244 248). 



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 

 covering the surface of the ovum. The villi, if really formed of mesoblast 

 covered by epiblast, have the true structure of chorionic villi ; and can 

 hardly be compared to the early villi of the dog which are derived from the 

 subzonal membrane, and still less to those of the rabbit formed from the 

 zona radiata. 



Unless all the early ova so far described are pathological, it seems to 



