MAMMALIA. 221 



becomes the epiblast in a closely allied species 1 To my mind it is not 

 conceivable, and I am reduced to the hypothesis, put forward by Hensen, 

 that in the course of the attachment of the ovum to the wall of the uterus 

 a rupture of walls of the blastodermic vesicle takes place, and that they 

 become completely turned inside out. It must be admitted, however, that 

 in the present state of our knowledge of the development of the ovum on 

 the seventh and eighth days it is not possible to frame a satisfactory 

 explanation how such an inversion can take place. 



The Hlimail Embryo. Our knowledge as to the early development 

 of the human embryo is in an unsatisfactory state. The positive facts 

 we know are comparatively few, and it is not possible to construct from 

 them a history of the development which is capable of satisfactory com- 

 parison with that in other forms, unless all the early embryos known are 

 to be regarded as abnormal. The most remarkable feature in the develop- 

 ment, which was first clearly brought to light by Allen Thomson in 1839, 

 is the very early appearance of branched villi. In the last few years 

 several ova, even younger than those described by Allen Thomson, have 

 been met with, which exhibit this peculiarity. 



The best-]>reserved of these ova is one described by Reichert (No. 237). 

 This ovum, though probably not more than thirteen days old, was com- 

 pletely enclosed by a decidua refiexa. It had (fig. 164 A and B) a 

 flattened oval form, measuring in its two diameters 5 "5 mm. and 3 '5 mm. 

 The edge was covered with branched villi, while in the centre of each of 

 the flattened surfaces there was a spot free from villi. On the surface 



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.) 



adjoinin;? the uterine wall was a darker area (e) formed of two layers of 

 cells, which is interpreted by Reichert as the embryonic area, while the 

 membrane forming the remainder of the ovum, including the branched 

 villi, was stated Vjy Reichert to be com[>osed 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 



