GENERATION. 



453 



when it arrives in the uterus has also a similar 

 external envelope, which has received in man 

 and most animals the general appellation of 

 chorion. Baer is of opinion that the chorion 

 exists ready formed in the ovulum of the 

 ovary ; but his observations appear to us as 

 yet insufficient to prove this point, and we feel 

 inclined rather to adopt the view of Valentin, 

 who holds that it is probable that the chorion 

 is added to the ovulum after it has left the 

 Graafian vesicle, that is, during its passage 

 from the ovary to the uterus, somewhat in the 

 same manner as the albumen or shell is added 

 to the egg of the common fowl in its passage 

 through the oviduct. The analogy of all ovi- 

 parous animals is at least strongly in favour of 

 such a view of the mode of the production of 

 the chorion or external envelope ; while on the 

 other hand we ought not to lose sight of the 

 fact that though the external envelope or cho- 

 rion occupies the same position as the external 

 covering of the eggs of oviparous animals, its 

 structure and functions are very different, for 

 almost in every quadruped the chorion serves 

 important purposes in establishing that more 

 intimate union peculiar to viviparous animals, 

 which is formed between the ovum and uterus 

 in the placenta or analogous structure. 



It is only in the dog and rabbit that the ova 

 have hitherto been traced by actual observation 

 in the whole course of their progress through the 

 Fallopian tubes from the ovary to the uterus. 

 These observations we owe chiefly to the care- 

 ful researches of Cruikshank, Prevost and 

 Dumas, Baer, and Coste. In regard to other 

 animals we have only a few detached observa- 

 tions in some of them, and in the human 

 species the ova have never been observed in the 

 Fallopian tubes, nor indeed for some time 

 after they must have entered the uterus. We 

 do not therefore know, with any degree of cer- 

 tainty, at what distance of time after sexual 

 union the ovum passes into the uterus of the 

 human female. Great difficulties attend the 

 elucidation of this point. In the first place, 

 we are opposed by the impossibility, in the 

 greater number of cases in which we may hap- 

 pen to obtain a pregnant uterus for investiga- 

 tion, of knowing accurately the age of the 

 product or the time at which impregnation has 

 occurred; and in the second place, we are 

 here deprived of the assistance derived in 



In the Quadruped. 



1. The ovary contains 



2. Graafian vesicles 

 which are filled with 

 fluid, granules, and 

 the proligerous disc, 

 in the centre of which 

 is placed 



3. The ovulum or vesi- 

 cle of Baer, consisting 

 of 



4. A yolk, on the sur- 

 face of which is 



5. A germinal membrane, 

 in the middle of which 

 is placed 



6. The germinal vesicle 

 or vesicle of Pnrkinje. 



In the Bird. 



1. The ovary contains 



2. Capsules entirely filled 

 with ovula, there being 

 no intervening fluid or 

 proligerous disc. 



3. The ovula or yolks, 

 consisting of 



4. A yolk. 



5. A germinal membrane 

 or cicatricula with the 



6. Vesicle of Purkinje in 

 its centre. 



many other parts of our subject from analogical 

 evidence by the wide discrepancies we find 

 among animals in respect to the period of the 

 arrival of the ova in the uterus ; for there does 

 not appear to be any exact correspondence yet 

 shewn between the time at which this happens 

 and the length of duration of utero-gestation. 

 It may be well, however, to endeavour to 

 form an approximative opinion. In the rabbit, 

 although ova are known frequently to be dis- 

 charged from the Graafian vesicles on the se- 

 cond day after sexual union, they are in general 

 not detected in the uterus before the third or 

 fourth day, and frequently not before the fifth 

 or sixth, at which time they appear as vesicles 

 a little more than a line in diameter, lying un- 

 attached in the upper part of the cornua of the 

 uterus.* 



In the dog ova have been observed in the 

 Fallopian tubes on the eighth day, but they 

 have not been found in the uterus before the 

 twelfth day. In the cat we have found ova of 

 the size of peas beginning to be attached to 

 the uterus at the twelfth day, and in both the 

 cat and dog we think it probable from the size 

 of the ova that they had already been in the 

 uterus for at least one day, so that the tenth 

 or eleventh day may be regarded as the time 

 when ova generally appear in the uterus of 

 these animals. 



Haller and Kuhlemannf never found an ovum 

 in the uterus of the sheep till the seventeenth 

 day after copulation, and our own observations 

 on both the sheep and sow agree precisely 

 with theirs. Hausmann never found the ova 

 in the uterus of the sow before the period of 

 four weeks after conception, and those of the 

 bitch before three weeks ; but here we must 

 caution the reader against the error of sup- 

 posing that in the sheep and some other 

 animals, because the ova have not been ob- 

 served in the uterus, they do not actually 

 exist there previous to a certain date ; for the 

 large size of the ovum and its membranes, as 

 well as the state of the foetus, which though 

 small is already somewhat developed, entitle 

 us to conclude that the ovum of the sheep 

 must have been some time in the uterus. The 

 recent interesting observations by M. Coste 

 have thrown great light upon this subject, he 

 having detected the ova of the sheep so early 

 as five days after conception. In the cow also, 

 in which the period of gestation is nearly twice 

 the length of that in the sheep, the ovum 

 seems to arrive almost as early in the uterus, 

 if we may judge from the state of advancement 

 of the foetus at an early period.]; 



* M. Coste has shewn that there is considerable 

 variety in respect to the time at which the ova 

 descend in the rabbit, and thus very reasonably 

 accounts for the difference one generally finds in 

 the state of advancement of the ova in the preg- 

 nant uterus. 



t Vide Kuhlemann's Observ. quaed. circa nego- 

 tium generations in ovibus fact. Gott. 1753. 



J Immediately after the arrival of the mammi- 

 ferous ovum in the uterus it increases in bulk with 

 amazing rapidity, and its membranes being thus 

 suddenly dilated become in consequence very weak 

 and thin ; so tender indeed are they, that if they 



