24 



ANATOMY OF VERTEBKATES. 



lines a b ; the dark fine lines represent injected capillaries, con- 

 tinued directly from the fibrous walls of the uterus, a c. Such 

 pulpy vascular substance, compared by John Hunter to 

 ' coagulable lymph,' 1 is rapidly formed, readily shed, speedily 

 renewed : it grows with the needs of the growing embryo or 

 foetus, as the medium for bringing into requisite relation with the 

 circulating system of such the mother's blood. 



In the uterus with a non-caducous and well-organised lining 

 membrane the chorion or outer coat of the ovum continues 

 smooth and unvascular, at least, until the foetus and appendages 

 have advanced to the degree shown in fig. 569. But in the 

 deciduate type of lining substance there is a reciprocal preparation 



of the chorion for intimate 

 connection therewith in the 

 form of villi, or long fila- 

 mentary vascular proces- 

 ses, extending from more 

 or less of its outer surface, 

 fig. 571, cho. 



In the very small pro- 

 portion of the placental 

 series in which the early 

 phases of development in 

 utero have been traced, so 

 much diversity has been 

 recognised as to warn 

 against too hasty generali- 

 sations. 



In the Rabbit, before the ovum enters the uterus, it has received 

 from the oviduct additional layers, ib. A, B, which, either in com- 

 bination with, or substitution for, the hyalinion, become the 

 medium of applying the capillaries of the foetus to those of the 

 mother continued into the decidua. When the foetal appendages 

 have attained, in this Rodent, the stage they exhibit in the Mar- 

 supial, fig. 569 — viz. the formation of the amniotic bag, fig. 571, a, 

 the inclosure of the unconverted germ-mass, or yolk, by a vascular 

 vitellicle, b, and the out-budding of a small allantois, c — the em- 

 bryo is by no means so far advanced. The ventral parietes of the 

 thoracic-abdominal cavity are not formed, the bilocular heart is 



1 In the indication of the various definable parts of this deciduous substance a voca- 

 bulary of terms has been created ; e. g. ' non-placental uterine mucous membrane,' ' ute- 

 roplacental mucous membrane,' ' persistent,' or 'non-deciduous serotina.' 'deciduous 

 serotina,' ' fimbriae of decidua serotina,' ' membrana decidua,' ' decidua reflexa,' ' rags,' 

 ' tags,' &c. 



Uterine ova, Rabbit, cccviii. 



