222 HUMAN OVUM. 



has a more complicated structure than Reichert is willing to admit. These 

 authors do not however agree amongst themselves, but from Kollmann's 



en -^ y^„^ 



riJfi 



Fig. 165. Three early human embryos. (Copied from His.) 



A. An early embryo described by His from the side. am. amnion ; urn. umbilical 

 vesicle ; ch. chorion, to which the embryo is attached by a stalk. 



B. Embryo described by Allen Thomson about 12 — 14 days. inn. umbilical 

 vesicle; md. medullary groove. 



C. Young embryo described by His. nm. umbilical vesicle. 



description, which appears to me the most satisfactory, it is probable that it 

 is composed of an outer epithelial laj'er, and an iuner layer of connective 

 tissue, and that the connective tissue extends at a very early period into 

 the villi; so that the latter are not hollow, as Reichert supposed them 

 to be. 



The villi, which at first leave the flattened [)oles free, seem soon to 

 extend first over one of the flat sides, and finally over the whole ovum 

 (fig. 164 C). 



Unless the two-layered region of Reichert's ovum is the embi-yonic 

 area, nothing which can clearly be identified as an embryo has been 

 detected in these early ova. In an ovum described by Breus (No. 228), 

 and in one described long ago by Wharton-Jones a mass found in the 

 interior of the egg may perhaps be interpreted (His) as the remains of the 

 yolk. It is, however, very probable that all the early ova so far discovered 

 are more or les-s pathological. 



The youngest ovum with a distinct embryo is one described by His 

 (No. 232). This ovum, which is diagrammatically represented in fig. 

 168 in longitudinal section, had the form of aai oval vesicle completely 

 covered by villi, and 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-sack was 

 attached to the inner side of the flatter wall of the vesicle by a stalk, 



