iM AM MA LI A. 



219 



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

 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^ , 

 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 inwai-ds, and the 

 ventral surface outwards, 

 and the ordinary position 

 of the layers is comj)letely 

 inverted. 



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

 mesoblast arises in connection with the primitive streak in the mariner 

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

 inner side of the outer hypoblastic wall of the ^g^, 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 allantois. 

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

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

 \tnderstood by a description of this figure. 



It is seen to consist of two vesicles, (1) an outer larger one (/i) — the 

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

 cular connection at niin, 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 hypoblast (/i) which is either pierced or 



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

 cylinder from the body which be 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 longitumnal section 

 through the embryo of a guinea-pig with its 

 MEMBRANES. (After Schafor.) 



e. epiblast; /(.hypoblast; jh'. amniotic mesoblast ; 

 m". splanchnic mesoblast; am. amnion; ci'. cavity 

 of amnion; aZZ. allantois; f. rudimentary blastopore; 

 mc. cavity of vesicle continuous with body-cavity; 

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

 vascular uterine tissue perforates hypoblast of blas- 

 todermic vesicle; vt. uterine vascular tissue; I. limits 

 of uterine tissue. 



