838 MAMMALIA. 



The ovaria (fig. 422, a) entirely lose all traces of their original racemose 

 condition ; for now the quantity of granular matter enclosed along with 

 the germ in each Graafian vesicle, the last remnant of the yelk, has 

 become almost inappreciable, and the little ovarian ovules are enclosed 

 in a dense parenchymatous substance enveloped by a smooth albu- 

 gineous tunic. The Fallopian tubes (6) correspond, in the smallness of 

 their diameter, with the minuteness of the globules they are destined 

 to convey from the ovaries into the uterine receptacle ; and lastly, the 

 excretory canal of the bladder (d) becomes quite separated from the 

 vagina (e), and the anal and generative apertures are found completely 

 distinct from each other. 



(2485.) After the above brief sketch of the anatomy of the organs of 

 generation in the higher Mammalia, it now remains for us to trace the 

 development of the germ from the moment of impregnation to the birth 

 of the foetus, and observe in what particulars placental generation 

 differs from the oviparous and ovoviviparous types already described. 

 In the viviparous or placental Mammifer the effect of impregnation is 

 the bursting of one or more of the Graafian 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 mem- 

 branes composing them, when once ruptured, are speedily removed by 

 absorption ; but in the Mammal this is not the case, and a cicatrix 

 remains permanently visible upon the surface of the ovary, indicating 

 where the rupture has occurred : such cicatrices are known by the 

 name of corpora lutea. 



(2486.) On the rupture of the ovarian ovisac, the vesicle of Purkinje, 

 or the essential germ, accompanied only by a most minute quantity of 

 granular fluid, or yelk, is taken up by the fimbriated extremity of the 

 Fallopian tube, and conveyed into the interior of the uterus, where its 

 development commences. Observations are wanting to teach us pre- 

 cisely 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 exactly comparable to what occurs in the egg of 

 the Bird, already minutely described in the last chapter (2110 etseq.}, 

 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, tobe afterwards nourished 

 in the pouch of its mother from materials derived from the breast. 



(2487.) But precisely at that point of development where the Mar- 

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

 the functions both of the vitellicle and of the allantoid apparatus become 

 no longer efficient either for nutrition or respiration, a third system of 

 organs is developed in the placental Mammifer, whereby a vascular 



