MAMMALIA. 



265 



At the time when the ovum first becomes fixed it will be remembered 

 that it resembles the early blastodermic vesicle of the Rabbit, and it is 

 natural to suppose that the apparently hypoblastic mass attached to 

 the inner wall of the vesicle 

 becomes the solid body at the 

 end of the egg-cylinder. This 

 appears to be BischofFs view, 

 but, as shewn above, the solid 

 mass is really the epiblast ! 

 Is it conceivable that the hypo- 

 blast in one species becomes 

 the epiblast in a closely allied 

 species? 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 rup- 

 ture of walls of the blasto- 

 dermic vesicle takes place, and 



yk. inverted yolk-sack (umbilical vesicle) 



that they become completely formed of an external hypoblastic layer (shaded) 



and an internal vascular layer (black). At the 

 end of this layer is placed the sinus terminalis ; 

 all. allantois ; pi. placenta. 



The external shaded parts are the uterine 



FIG. 163. DIAGRAMMATIC LONGITUDINAL 

 SECTION OF AN OVUM OF A GUINEA-PIG AND THE 

 ADJACENT UTERINE WALLS AT AN ADVANCED 

 STAGE OF PREGNANCY. (After Bischoff.) 



turned inside out. It must be 



admitted, however, that in the 



present state of our knowledge 



of the development of the o- walls. 



vum on the seventh and eighth 



days it is not possible to frame a satisfactory explanation how such an 



inversion can take place. 



The Human Embryo. Our knowledge as to the early develop- 

 ment 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-preserved 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 reflexa. 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 adjoining 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 



