456 



GENERATION. 



maly ; but it may be stated as a general rule 

 that this does not occur in oviparous animals, 

 and more especially in birds, in which a con- 

 tinued supply of fresh air around the shell is 

 necessary to promote incubation, and we do 

 not know of any examples of truly oviparous 

 animals in which the foetus has been formed in 

 an egg accidentally retained within the body of 

 the parent. In none of those which we have 

 observed was there any appearance of foetal 

 formation. 



It is possible that some irregularities in the 

 position of the ovum of mammalia during ges- 

 tation may receive an explanation from mecha- 

 nical disturbances similar to those we have now 

 mentioned in birds ; for supposing that in a 

 viviparous animal the ovum does not gain the 

 uterus or usual place of its abode during gesta- 

 tion, development of the foetus still takes place. 

 In those instances in which a foetus is formed 

 in the region of the ovary, or in what are termed 

 ovarian conceptions, for example, it is not pro- 

 bable that the ovum is ever developed in the 

 ovary itself without the bursting of the Graafian 

 vesicle : it may be fixed close to the ovary, but 

 it is always independent of that body. After 

 the Graafian vesicle has burst, the ovum may 

 be supposed either not to have been received 

 in the Fallopian tube, or, after having entered 

 that passage, to have been expelled from it by 

 an inverted action of its muscular fibres or other 

 causes. Fecundated by the contact of some of 

 the seminal fluid which has reached so far 

 into the Fallopian tube, the ovum remains in 

 the neighbourhood of the ovary, has a cyst 

 formed round it, and becomes organically 

 united to the ovary or parts in its vicinity by 

 structures similar to those which unite the ovum 

 to the inner surface of the gravid uterus ; for 

 the bloodvessels of the mother which run into 

 the cyst enlarge and form a placenta by their 

 union and intermixture with those of the foetus, 

 and thus for a considerable time (amounting 

 sometimes to four or five months) this ovarian 

 or extra-uterine gestation is carried on. 



In other instances of misplaced gestation, the 

 ovum seems to have been arrested in its course 

 when more or less advanced in the Fallopian 

 tube ; but here also the parts are susceptible of 

 all those remarkable changes and growth which 

 favour the development of the foetus in the 

 ovum. We mention these instances of extra- 

 uterine gestation with the view of directing the 

 reader's attention to an inference which may be 

 drawn from them, viz. that all those changes 

 of growth upon which the development of the 

 ovum in viviparous animals depends may be 

 regarded rather as belonging to the ovum itself 

 than as resident in the uterus alone. It is 

 worthy of remark, however, that in ovarian and 

 tubular conceptions the decidua is formed 

 within the uterus, nearly in the same manner 

 as if the ovum had descended in the natural 

 way into its cavity ; from which we may infer 

 that the production of the decidua is to be re- 

 garded as one of a series of changes induced by 

 conception in the internal genital organs, and 

 occurring independently of one another, rather 

 than as the effect of any stimulation from 



the ovum, as some have supposed. Such a 

 decidua in fact may be compared to the sub- 

 ventaneous ovum of the bird. 



Very little is as yet known as to the physical 

 circumstances (independent of malformation of 

 the organs) which may give rise to misplaced 

 gestation ; and this is not a subject which we 

 can hope to have illustrated by observation or 

 experiment. One or two cases are on record, 

 however, from which it might appear possible 

 that a violent disturbance of the body soon after 

 sexual union may be a cause of misplacement 

 of the ovum. Burdach mentions instances of 

 this kind : one of a cow gored by the horns of 

 another soon after copulation, and two instances 

 of the human female in which sudden fright 

 in the same circumstances was followed by 

 ovarian conception.* 



In endeavouring to apply such mechanical 

 explanations, we ought, not to forget that in by 

 far the greater number of cases sudden motion 

 does not appear to disturb the natural perform- 

 ance of all those actions by which the ovum is 

 securely lodged in the uterus in the natural 

 way. 



Circumstances influencing the liability to 

 conception. The circumstances which influence 

 the liability of the female to conception are so 

 various and so little determined that our re- 

 marks on this subject must be very short. 



The healthy condition of the female is of 

 course an important circumstance in reference 

 to conception, but we do not know in how far 

 a robust constitution or high state of health is 

 favourable or the reverse to the occurrence of 

 conception. Some women, it would appear, 

 (perhaps those of a spare habit of body and 

 languid powers of constitution) are most liable 

 to fall with child when in their strongest and 

 best state of health, while weakness in others 

 seems to induce conception. Among animals 

 it is known that high feeding sometimes pre- 

 vents pregnancy, and the same is the effect of 

 the opposite extreme of starvation. 



The regularity of the menstrual discharge is 

 one of the most important circumstances which 

 favours the liability of women to conception ; 

 perhaps more from its being an indication of 

 the general healthy state of the generative or- 

 gans than from any influence exerted by the 

 menstrual change itself. Many circumstances, 

 however, seem to render it probable that women 

 are more liable to conception within a few days 

 after the cessation of the menstrual flow than at 

 any other period of the interval, and accordingly 

 there are many accoucheurs who regulate their 

 calculations of the time of birth from this cir- 

 cumstance, dating the commencement of utero- 

 gestation from a period within a week after the 

 cessation of the last menstrual discharge. We 

 do not know with certainty upon what circum- 

 stances this influence of the menstrual function 

 depends ; but it seems reasonable to suppose 

 that it is connected with that state of excitement 

 and sanguineous congestion in the ovaries and 



* See Lallemand's Observat. Patholog. 1818, 

 and Diet, des Scien. Med. xix. ; also Grasmeyer 

 de conceptu et fecundatione humana, 1789. 



