MAMMALIA. 263 



becomes hollow, and is filled with a fluid not coagulable in alcohol, while the 

 body within its apex remains unaltered till the tenth day. 



On this day a cavity develops in the interior of this body which at the 

 same time enlarges itself. The greater part of its wall next attaches itself 

 to the free end of the cylinder, and becomes considerably thickened. The 



FIG. 162. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE EMBRYO OF 



A GUINEA-PIG WITH ITS MEMBRANES. (After Schafer.) 



e. epiblast ; h. hypoblast ; in', amniotic mesoblast ; in" . splanchnic mesoblast ; 

 am. amnion ; ev. cavity of amnion ; all. allantois ; f. rudimentary blastopore ; me. 

 cavity of vesicle continuous with body cavity; mm. mucous membrane of uterus; 

 m'm'. parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle ; 

 vt. uterine vascular tissue ; /. limits of uterine tissue. 



remainder of the wall adjoining the cavity of the cylinder becomes a com- 

 paratively thin membrane. At the free end of the cylinder there appears on 

 the thirteenth day an embryonic area similar to that of other Mammalia. 

 It is at first round but soon becomes pyriform, and in it there appear a 

 primitive streak and groove ; and on their appearance it becomes obvious 

 that the outer layer of the cylinder is the hypoblast^, instead of, as in all 

 other Mammalia, the epiblast ; and that the epiblast is formed by the wall of 

 the inner vesicle, i.e. the original solid body placed at the end of the cylinder. 

 Thus the dorsal surface of the embryo is turned inwards, and the ventral 

 surface outwards, and the ordinary position of the layers is completely 

 inverted. 



1 According to Hensen the hypoblast grows round the inside of the wall of the 

 cylinder from the body which he regards as the ovum. The original wall of the 

 cylinder persists as a very thin layer separated from the hypoblast by a membrane. 



