336 



THE MAMMALIAN EMBRYO. 



[CHAP. 



THE HUMAN OVA DURING EARLY STAGES OP DEVELOPMENT. 

 (From Quain's Anatomy.) 



A. and B. Front and side view of an ovum figured by Keichert, 



supposed to be about thirteen days. e. embryonic area. 

 C. An ovum of about four or five weeks shewing the general 



structure of the ovum before the formation of the placenta. 



Part of the wall of the ovum is removed to shew the embryo 



in situ. (After Allen Thomson.) 



early ova. In an ovum described by Breus, and in one 

 described long ago by Wharton-Jones, a mass found in 

 the interior of the ovum may perhaps be interpreted 

 (His) as the remains of the yolk. It is, however, very 

 probable that all the early ova so far obtained are 

 more or less pathological. 



The youngest ovum with a distinct embryo is one 

 described by His. This ovum, which is diagrammati- 

 cally represented in Fig. Ill in longitudinal section, 

 had the form of an oval vesicle completely covered by 

 villi, being about 8*5 mm. and 5*5 mm. in its two 

 diameters, and flatter on one side than on the other. 

 An embryo with a yolk-sac was attached to the inner 

 side of the flatter wall of the vesicle by a stalk, which 

 must be regarded as the allantoic stalk; the embryo 



