MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 339 



Aves, in order to show the extent and nature of the correspondence above alluded to, 

 and at the same time to form a probable opinion of the source whence the contents 

 of the umbilical vesicle had been in the present instance derived. In Birds, the ma- 

 terial of the yolk is added to the efficacious part of the ovum (the vesicle of Purkinje,) 

 while in the ovary, and the ovum consequently acquires a considerable size from the 

 accumulation of the vitelline matter before it passes into the oviduct, which presents 

 a corresponding capacity for its reception. De Graap long ago observed, that small as 

 were the perfected ova in the ovaries of quadrupeds, in comparison with those in the 

 ovaries of birds, yet the vesicle which was discovered after impregnation in the Fallo- 

 pian tube was still more minute than the ovarian vesicle from which he conjectured 

 that it had escaped. Thus, in his celebrated experiments on the Rabbit, he observes of 

 the female organs seventy-two hours after impregnation : " In altero autem testiculo 

 quatuor foUiculos invenimus, quorum tres aliquantulum magis lucidi, minorique fora- 

 mine pertusi videbantur, in quorum etiam medio tantillum limpidissimi liquoris ad- 

 invenimus : at quartus follicuius obscurior erat, nee quicquam liquoris in se contine- 

 bat ; quamobrem ovum ex hoc folliculo elapsum suspicabamur, qua de causa ejusdem 

 lateris cornu et oviductum perscrutati sumus, ac unicum tantum ovum in ipso cornu 

 principio deprehendimus, perpusillis alterius lateris ovis simillimum =*." 



Mr. Cruikshank, who repeated these experiments of De Graaf, succeeded in de- 

 tecting the ovulum in the Fallopian tube of the Rabbit on the fourth day after impreg- 

 nation, and believes that he saw it in the corpus luteum, as he terms the Graafian 

 follicle, observing, " The pouting part I believe is the ovum, and stands upon the 

 top of the corpus luteum : it is very vascular, particularly at its basis ; but as soon as 

 perfect, or ready for expulsion, carries no red blood : it continues to grow of itself in 

 utero, without adhering to the uterus for two or three days, then takes root and be- 

 comes very vascular -j-." 



Any doubts that might still remain as to the pre-existence of the ovulum in the 

 Graafian follicle of the Mammalia have been in great measure dispelled by the more 

 recent and accurate observations of Von Baer, who considers it identical with the 

 pellucid vesicle a short time before discovered by Purkinje in the ovary of the Fowl, 

 around which the yolk accumulates to form the ovum prior to its passage into the 

 oviduct. 



Escaping then from the ovary without being inclosed within this superadded ma- 

 terial, the ovulum in Mammalia passes along the oviduct or Fallopian tube, which is 

 consequently of small diameter. It increases in size, according to De Graaf and 

 Von Baer, as it passes along the tube, by imbibition of nutrient material ; and this 

 mode of increase goes on rapidly after it has reached the uterus. The granules con- 

 tained in the so-imbibed fluid accumulate at its periphery, and constitute the germinal 

 membrane ; and while the ovum yet floats freely in the uterus, villi, in the ordinary 

 Mammalia, are observed to shoot out from the chorion or external membrane. But 

 * De Graaf, Opera Omnia, p. 400. f Philosophical Transactions, vol. Ixxxvii. p. 206. 



