MAMMALIA. 



I'll) 



h 



in m 



tn m 



/n m 



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 con- 

 siderably thickened. The 

 remainder of the wall ad- 

 joining the cavity of the 

 cylinder becomes a com- 

 paratively thin membrane. 

 At the free end of the cy- 

 linder 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 ob- 

 vious that the outer layer of 

 the cylinder is the hypoblast 1 , 

 instead of, as in all other 

 Mammalia, the epiblast ; and 

 //nit the epiblast is formed by 

 the wall of the inner vesicle, 

 i.e. the original solid bod// 

 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. 



The previously cylindrical egg next assumes a spherical form, and the 

 ruesoblast arises in connection with the primitive streak in the manner 

 already described. A splanchnic layer of mesoblast attaches itself to the 

 inner side of the outer hypoblastic wall of the egg, a somatic layer to the 

 epiblast of the inner vesicle, and a mass of mesoblast grows out into the 

 cavity of the larger vesicle forming the commencement of the allaiitois. 

 The general structure of the ovum at this stage is represented on tig. 162, 

 copied from Schiifer ; and the condition of the whole ovum will best be 

 understood by a description of this figure. 



It is seen to consist of two vehicles, (1) an outer larger one (h) the 

 original egg-cylinder united to the mesometric wall of the uterus by a vas- 

 cular connection at in' in, and (2) an inner smaller one (ev) the originally 

 solid body at the free end of the egg-cylinder. The outer vesicle is formed of 

 (1) an external lining of columnar hypoblasfc (h) which is either pierced or 



1 According to Heusen the hypoblast grows round the inside of the wall of tlie 

 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. 



FIG. 162. DIAGRAMMATIC LONGITUDINAL SECTION 

 THROUGH THE EMBRYO OF A (JUINEA-PIG WITH ITS 

 MEMBRANES. (After Schafer.) 



e. epiblast; //.hypoblast; in. amuiotic mesoblast ; 

 m". splanchnic mesoblast; am. amnion; cr. cavity 

 of amuion: all. allantois; /. rudimentary blastopore; 

 inc. cavity of vesicle continuous with body-cavity; 

 nun. mucous membrane of uterus; mm', parts where 

 vascular uterine tissue perforates hypoblast of blas- 

 toderrnic vesicle ; vt. uterine vascular tissue; 7. limits 

 of uterine tissue. 



