860 FERTILISATION OF THE OVUM. 



(partial closure of the vagina or vulva), the penis has not passed into the vagina during coition, 

 prove that the sj>ermatozoa can traverse the whole length of the vagina, and pass into the 

 uterus. 



438. FERTILISATION OF THE OVUM. The ovum is fertilised by one 

 spermatozoon passing into it. 



Swammerdam (t 1685) proved that contact of the semen with the ovum was necessary for 

 fertilisation. Snallanzani (1768) proved that the fertilising agent was the spermatozoa, and 

 not the clear filtered fluid part of the semen, and that the spermatozoa, even after being 

 enormously diluted, were still capable of action. Martin Barry (1850) was the first to observe 

 the entrance of a spermatozoon into the ovum of the rabbit. This occurs pretty rapidly, by a 

 boring movement through the vitelline membrane (Leuckhart). The entrance is effected either 

 through the porous canals or the micropyle (Kcber, p. 850). 



Theories. As to the manner in which the spermatozoon affects the ovum, there are great 

 differences of opinion. Aristotle compared it to an action like that of rennet on milk ; Bischoff, 

 to that of yeast on a fermentable mass, i.e., to a catalytic action. These theories, however, are 

 quite unsatisfactory, as we know that the unfertilised ova of the hen, rabbit (Henseri), pig 

 (liischqf), salpa [Kuppfer) (but not the frog Pflilger) can undergo the initial stages of develop- 

 ment as far as the stage of cleavage, and the star-fishes even as far as the larval form (Greef). 



Place of Fertilisation. The place where fertilisation occurs is either the ovary \ 

 as indicated by the occurrence of abdominal pregnancy, or the Fallopian tube, and 

 the numerous recesses in the latter afford a good temporary nidus for the sperma- 

 tozoa. This view is supported by the occurrence of tubal pregnancy. Thus, the 

 spermatozoa must be able to pass through the Fallopian tube to the ovary, which 

 is probably brought about chiefly by the movements proper to the spermatozoa 

 themselves. It is uncertain whether the peristaltic movements of the uterus and 

 Fallopian tube are concerned in this process ; certainly ciliary movement is not con- 

 cerned, as the cilia of the Fallopian tube act from above downwards. When once 

 the ovum has passed unfertilised into the uterus, it is not fertilised in the uterus. 

 It is assumed that the ovum reaches the uterus within 2 to 3 weeks (in the bitch, 

 8 to 14 days). 



Twins occur in 1 in 87 preguancies, but oftener in warm climates ; triplets, 

 1 : 7C00 ; four at a birth, 1 : 330,000. More than six at a time have not been 

 observed. The average number of pregnancies in a woman is 4J. 



Superfecundation. By this term is understood the fertilisation of two ova at the same 

 menstruation, by two different acts of coition. Thus, a mare may throw a foal and a mule, 

 after being covered first by a stallion and then by an ass. A white and a black child have been 

 born as twins by a woman. 



Superfcetation is when a second impregnation takes place at a later period of pregnancy, as 

 in the second or third month. This, however, is only possible in a double uterus, or when 

 menstruation persists until the time of the second impregnation. It is said to occur frequently 

 in the hare. 



Hybrids are produced when there is a cross between different species (horse, ass, zebra dog, 

 jackal, wolf goat, ibex goat, sheep species of llama camel, dromedary tiger, lion species 

 of pheasant -goose, swan carp, crucian species of butterflies). Most hybrids are sterile, 

 especially as regards the formation of properly formed spermatozoa ; while the hybrid females 

 are for the most part fertile with the male of both parents, e.g., the mule ; but the characters 

 of the offspring tend to return to those of the species of the parents. Very few hybrids are 

 fertile when crossed by hybrids. In many species of frogs the absence of hybrids is accounted 

 for by the mechanical obstacles to fertilisation of the ova. 



Tubal Migration of the Ovum. Under exceptional circumstances, the ovum 

 discharged from a ruptured Graafian follicle passes into the Fallopian tube of the 

 other side, as is proved by the occurrence of tubal pregnancy and pregnancy of an 

 abnormal rudimentary horn of the uterus, in which case the true corpus luteum 

 is found on the other side of the ovary. This is spoken of as "external migration" 

 (kussmaul, Leopold). This observation coincides with experiment, as granular 

 fluids, e.y., China-ink, when injected into the peritoneal cavity, pass into both 

 Fallopian tubes, and are carried by the ciliated epithelium to the uterus (Pinner). 

 In animals, with a double uterus with two orifices, the ova may migrate through 



