472 THE HUMAN EMBRYO. 



segmentation in the lower part of the oviduct, and not entering 

 the uterus until from eight to ten days, or perhaps longer, after 

 the time of discharge from the ovary. 



2. The Second Week. 



Of ova or embryos which are believed to belong to the end 

 of the second week a few examples have been described. These 

 are of great interest, although there is room for doubt in some 

 of these cases whether the specimens can be regarded as perfectly 

 normal. 



Reichert's ovum. The best known instance is an ovum de- 

 scribed by Reichert, and believed to be of about the twelfth or 

 thirteenth day. This ovum, which is represented four times the 



FIG. 172. FIG. 173. 



FIGS. 172 and 173. Front and side views of Reichert's Ovum. (From Kolliker, 

 after Reichert.) x 4. 



natural size in Figs. 172 and 173, was found, in situ, in the 

 uterus of a woman who had committed suicide, and gave every 

 indication of being perfectly normal. 



The ovum was a vesicular body, lenticular in shape, and 

 measuring 5*5 mm. across its greater diameter, and 3 -3 mm. 

 from side to side. Of the two surfaces, the one turned towards 

 the wall of the uterus, the upper one in Fig. 173, was more 

 convex than the opposite surface, which faced towards the cavity 

 of the uterus. The margin of the vesicle was thickly fringed 

 with villi, the largest of which were 0'2 mm. long, and slightly 

 branched; the middle portions of both surfaces were smooth, 

 and devoid of villi ; and in the centre of the more convex or 

 uterine surface was a small circular spot (Fig. 172), 1-6 mm. 

 in diameter, and of a darker colour than the rest of the vesicle. 



The relations of the ovum to the uterus were as follows. The 



