728 MAMMALIA. 



enveloped by a smooth albugineous tunic. The Fallopian tubes 

 (b) correspond, in the smallness of their diameter, with the minute- 

 ness of the globules they are destined to convey from the ovaries 

 into the uterine receptacle ; and lastly, the excretory canal of the 

 bladder (cf) becomes quite separated from the vagina (e), and the 

 anal and generative apertures are found completely distinct from 

 each other. 



(858.) After the above brief sketch of the anatomy of the or- 

 gans of generation in the higher Mammalia, it now remains for us 

 to trace the developement of the germ from the moment of im- 

 pregnation to the birth of the fetus, and observe in what particu- 

 lars placental generation differs from the oviparous and ovo-vivipa- 

 rous types already described. In the viviparous or placental Mam- 

 mifer, the effect of impregnation is the bursting of one or more of 

 the Graqfian vesicles, and the escape of the contained germs from 

 the ovisacs wherein they were formed. In the Ovipara, owing to 

 the delicacy of the ovisacs, the vascular membranes composing them 

 when once ruptured are speedily removed by absorption ; but in 

 the Mammal this is not the case, and a cicatrix remains perma- 

 nently visible upon the surface of the ovary, indicating where the 

 rupture has occurred : such cicatrices are known by the name of 

 corpora lutea. 



(859.) On the rupture of the ovarian ovisac, the vesicle of Pur- 

 kinje, or the essential germ, accompanied only by a most minute 

 quantity of granular fluid, or yolk, is taken up by the fimbriated 

 extremity of the Fallopian tube, and conveyed into the interior of 

 the uterus, where its developement commences. Observations are 

 wanting to teach us precisely what are the first appearances of the 

 embryo ; but there is not the least doubt that the materials for its 

 earliest growth are absorbed in the cavity of the womb, and that 

 its formation from a blastoderm, or germinal membrane, is ex- 

 actly comparable to what occurs in the egg of the Bird, already 

 minutely described in the last chapter ( 699 et seq.), and that 

 in every particular, as relates to the growth and functions of the 

 vitelline or omphalo-mesenteric as well as of the amniotic systems, 

 the phenomena are the same as in the marsupial Mammal up to 

 the period when the young Marsupian is prematurely born, to be 

 afterwards nourished in the pouch of its mother from materials de- 

 rived from the breast. 



But precisely at that point of developement where the Marsu- 

 pial embryo is expelled from the uterus of its parent, namely, when 



