THE SECOND WEEK. 475 



coagulation product, is of the nature of connective tissue, and 

 therefore of mesoblastic origin. This latter point is. however, a 

 doubtful one. 



In the present state of our knowledge it is hardly possible 

 to make any satisfactory comparison between these early human 

 ova, and the stages already described as occurring in the rabbit. 

 The difficulty is much increased by the absence of detailed 

 histological description, and by the doubt as to whether the ova 

 are in all respects normal. It must also be borne in mind that 

 we are absolutely ignorant of the mode in which segmentation 

 of the human ovum, and the immediately succeeding stages are 

 effected ; and that great uncertainty still exists with regard to 

 the details of these processes in the rabbit. 



As, however, the several human ova of the stage in question 

 agree in a number of important points, and as, in the case of 

 Reichert's ovum, there is every reason for regarding the specimen 

 as normal, it is advisable to make such comparison as is possible 

 between these ova and the several stages of development of such 

 a Mammal as the rabbit. 



In the first place, the complete absence of any trace of an 

 embryo indicates that the stage is a very early one. In ova 

 which there are strong reasons for regarding as but one or two 

 days older, an embryo is present ; so that the stage represented 

 by Reichert's ovum may be described as one shortly before the 

 first appearance of the embryo, and as corresponding in this 

 respect with the blastodermic vesicle of a rabbit at about the 

 fifth or sixth day. 



In its vesicular character, the thinness of its walls, and the 

 presence of a central embryonal area of different constitution to 

 the rest of the wall, there are additional points of resemblance 

 between Reichert's ovum and" the blastodermic vesicle of a 

 rabbit of the sixth day, or of a dog in the early part of the 

 third week. There is also a close correspondence in actual size 

 between these stages in the three instances. 



Reichert was of opinion that this comparison was a true 

 one ; and the view is supported by His, who gives in illustra- 

 tion of it the diagrammatic section (Fig. 174). His considers 

 that the outer wall of the vesicle consists of epiblast only, and 

 that the hypoblast forms the inner circular patch of cells in the 

 embryonal area ; he also regards the central rounded mass of 



