STRUCTURE AND DEVELOPMENT OF THE OVUM. 447 



to the surface of the ovaries at the time of the discharge of the ova, 

 that these are received and conveyed to the uterus, instead of being 

 allowed (as in some of the lower animals) to fall into the abdominal 

 cavity. 



793. There are many cases among the lower classes, in which the 

 ovum is retained within the oviducts, so that the young comes into the 

 world. alive ; and there are a few in which, during this delay, it receives 

 a direct supply of additional nourishment from the fluids of its parent. 

 It is in the Mammalia, however, that we find the mostf remarkable and 

 complete provision for this purpose. Still, the lowest division of this 

 group approximates closely, in the type of its generative apparatus, to 

 the Oviparous Vertebrata ; for the oviducts of the Monotremata remain 

 distinct from each other, and terminate separately in the uro-genital 

 canal, each of them having first undergone dilatation into a uterine 

 cavity, so that these animals have two completely distinct uteri. In 

 the Marsupialia, there is a closer approximation of the two lateral sets 

 of organs on the median line ; for the oviducts converge towards one 

 another, and meet on the median line, but without coalescing ; so that 

 these animals have a true " double uterus," opening by two orifices into 

 the vaginal canal, a condition which is sometimes met with as a malfor- 

 mation in the Human female. The vaginal canal, however, is also 

 double ; which is less frequently observed in the Human species. The 

 two preceding orders constitute the sub-class of Implacental Mammals ; 

 the development of their ova within the uteri being cut short at a period 

 anterior to the formation of the placenta ( 818). As we ascend 

 through the series of Placental Mammals, we find the lateral coales- 

 cence of the uterine dilatations of the Fallopian tubes becoming more 

 and more complete. It first shows itself in the vagina, which is every- 

 where single, although a trace of separation into two lateral halves is 

 seen in the Mare, Ass, Cow, Pig, and Sloth, in which animals it is tra- 

 versed, in the virgin state, by a narrow vertical partition. In many of 

 the Rodentia, the uterus still remains completely divided into two 

 lateral halves, opening into the vagina by separate orifices ; whilst in 

 others, these coalesce at their lower portion, forming a rudiment of the 

 true " body" of the uterus of the Human female. This part increases 

 in the . more elevated Herbivora and Carnivora, at the expense of the 

 lateral ununited portions, which are now termed the " cornua ;" but 

 even in the lower Quadrumana, the uterus is someWhat cleft at its sum- 

 mit, and the "angles," into which the oviducts enter, form a consider- 

 able part of the whole organ. As we ascend through the Quadruma- 

 nous series towards Man, we find the "body" of the uterus increasing, 

 and the "angles" diminishing in proportion, until the original division 

 is completely lost sight of, except in the slight dilatation of the cavity 

 at the points at which the Fallopian tubes enter it. 



794. Having thus briefly noticed the most important characters of 

 the organs provided for the original production and for the subsequent 

 reception of the ova, we have now to inquire into the history of their 

 development. The essential structure of the ovule, or unfertilized egg, 

 appears to be the same in all animals. It consists externally of a 

 membranous sac, termed, from the nature of its contents, the vitelline 



