X.] THE HUMAN EMBRYO. 335 



The human embryo. Our knowledge as to the 

 early development of the human embryo is in an un- 

 satisfactory state. The positive facts we know are com- 

 paratively few, and it is not possible to construct from 

 them a history of the development which is capable of 

 satisfactory comparison 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 preserved of these ova is one described by 

 Reichert 1 . This ovum, though probably not more than 

 thirteen days old, was completely enclosed by a decidua 

 reflexa. It had (Fig. 109 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 ad- 

 joining the uterine wall was a darker area (e) formed of 

 two layers of cells. Nothing certain has been made out 

 about the structure of ova of this age. 



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. 109 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 



1 Abhandlungen der Konigl. Akad. d. Wiss. zu Berlin, 1873. 



