2 66 HUMAN OVUM. 



the remainder of the ovum, including the branched villi, was stated by 

 Reichert to be composed of a single row of epithelial cells. 



Whether or no Reichert is correct in identifying his darker spot as the 

 embryonic area, it is fairly certain from the later observations of Beigel and 

 Lowe (No. 228), Ahlfeld (No. 227), and Kollmann (No. 234) on ova nearly 

 as young as that of Reichert, that the wall of very young ova has a more 

 complicated structure than Reichert is willing to admit. These authors do 

 not however agree amongst themselves, but from Kollmann's description, 

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

 of an outer epithelial layer, and an inner 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. 



FIG. 164. THE HUMAN OVA DURING EARLY STAGES OF DEVELOPMENT. 



(From Quain's Anatomy.) 



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



The villi, which at first leave the flattened poles 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 embryonic 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 

 less 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 an oval vesicle completely covered by 

 villi, and about 8'5 mm. and $'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, which must be 



