VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 113 



tube, and conveyed into the uterus, soon assumes a visible vesicular form, and is 

 called an ovum. 6°. That these rudiments of the new animal, which for a time 

 manifested no arrangement of parts, afterwards begin to elaborate and evolve the 

 different organs of which the new animal is composed. To these facts we may 

 add, that the calyx or capsula which formed the parietes of the vesicles, thickens, 

 by which the cavity is diminished. This cavity, together with the opening through 

 which the foetal rudiments escaped becomes obliterated, and from the parietes of 

 these vesicles having acquired a yellowish hue, they are called corpora lutea. 



But though some important facts are clearly ascertained, there are others still 

 problematical. Physiologists are by no means agreed concerning the immediate 

 cause of conception. All admit the necessity of sexual intercourse. They acknow- 

 ledge too the necessity of some part of the female being affected by the direct con- 

 tact of a fecundating fluid, but what the precise part is which must receive the 

 stimulus, has hitherto been involved in mystery and doubt. Nor are they more 

 unanimous respecting the state or condition of the substance that passes from the 

 ovaries ; whether at the time of its expulsion it has a circumscribed vesicular cha- 

 racter, or whether it has no determined figure. De Graaf and Malpighi, in the 

 last century, and some respectable physiologists of the present day, adopt the first 

 opinion ; Haller and some others favour the last. 



The intention then of this essay is to explore the proximate cause of the impreg- 

 nation of animals, and to trace with more accuracy the visible effects of it from 

 their first appearance, till the rudiments of the foetus are lodged in the uterus, and 

 have assumed the proper characters of an ovum. As soon as these rudiments ma- 

 nifest that opaque spot, or " dim speck of entity," which is known to evolve the 

 foetus by regular and progressive steps ; another stage of the inquiry then com- 

 mences, viz. to trace the visible formation of the new animal through its whole 

 course ; but as this belongs rather to the economy of the foetus than the mother, 

 it is not intended to form any part of this paper. I perceive however, that I can- 

 not investigate the question of the proximate cause of impregnation in a satisfactory 

 way without first determining what are the evidences or proofs that impregnation 

 has taken place : this then necessarily becomes a preliminary question. I therefore 

 restrict my inquiry to the three following subjects. 1st. What are the evidences 

 of impregnation ? 2d. What is the proximate cause of impregnation ? And, 3d. 

 Under what form do the rudiments of the foetus pass from the ovary to the uterus ? 



^ 1 . What are the evidences of impregnation f — Tn order that I might bear evi- 

 dence of the truth, that a female has conceived before there are any vestiges of a 

 new animal, I examined with great attention the ovaries of some full-grown virgin 

 rabbits, and found, as De Graaf has represented, that there entered into their com- 

 position a series of cells containing a transparent colourless fluid. But in none of 

 them could I see any of those circumscribed substances, which, from their yellow 

 colour, are called corpora lutea. But when similar observations were made on 

 rabbits that had been impregnated at different periods, and the traces .of those co- 



VOL. xviii. Q 



