MAMMALIA. 



201 



Fig. 149. Section through the placenta and adjacent 

 PARTS OF A Eat one inch and a quarter long. (From 

 Huxley. ) 



a. uterine vein ; b. uterine wall ; c. cavernous portion of 

 uterine wall ; d. deciduous portion of uterus with cavernous 

 structure ; i. large vein passing to the foetal portion of the 

 placenta ; /. false chorion supplied by vitelline vessels ; 

 k. vitelline vessel ; I. allantoic vessel ; g. boundary of true 

 placenta ; e, m, rw, e. line of junction of the deciduate 

 and non-deciduate parts of the uterine wall. > 



Guinea-pig". The development of the Guinea-pig is dealt with else 

 where, but, so far as 

 its peculiarities per- 

 mit a compai'ison with 

 the Rabbit, the agree- 

 ment between the two 

 types appears to be 

 fairly close. 



The blastodermic 

 vesicle of the Guinea- 

 pig becomes complete- 

 ly enveloped in a 

 capsule of the uterine 

 wall (decidua reflexa) 

 (fig. 150). The epi- 

 thelium of the blasto- 

 dermic vesicle in con- 

 tact with the uterine 

 wall is not epiblastic, 

 but corresponds with 

 the hypoblast of the 

 yolk-sack of other 

 forms, and the meso- 

 Idast of the greater 

 part of the inner side 

 of this becomes richly 

 vascular (yk); the vascular area being bounded by a sinus terminalis. 



The blastodermic vesicle is so situated within its uterine capsule that the 

 embryo is attached to the part of 



it adjoining the free side of the 

 uterus. From the opposite side 

 of the uterus, viz. that to wliich 

 the mesometrium is attached, 

 there grow into the wall of the 

 blastodermic vesicle mimerous 

 vascular processes of the uterine 

 wall, which establish at this 

 point an organic connection be- 

 tween the two (pi). The blood- 

 vessels of the blastodermic ves- 

 icle (yolk-sack) stop s^hort im- 

 mediately around the area of 

 attachment to the uterus ; but 

 at a late period the allantois 

 grows towards, and fuses with 

 this area. The blood-vessels of 

 the allantois and of the uterus 

 become intertwined, and a disc- 

 like placenta more or less simi- 

 lar to that in the Rabbit be- 

 comes formed (pi). The cavity 

 of the allantois, if developed, vanishes completely 



Fig. 150. Diagrammatic longitudinal sec- 

 tion OF AN OVUM OF A GuiNEA-PIG AND THE 

 ADJACENT UTERINE WALLS AT AN ADVANCED STAGE 



OF PREGNANCY. (After Bischoff.) 



yk. yolk-sack (umbilical vesicle) formed of 

 an external hypoblastic layer (shaded) and an 

 internal mesoblastic vascular layer (black). At 

 the end of this layer is placed the sinus termi- 

 nalis ; all. allantois ; pi. placenta. 



The external shaded parts are the uterine 

 walls. 



