SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 247 



lion from the ordinary relations is figured above, where two distinct 

 animals [Diplozoon) join in almost life-long union. 



In many cases again, especially in bony fishes, there is a sexual attrac- 

 tion between male and female, but without any copulation. The female, 

 accompanied by 'her mate, deposits ova, which he thereupon fertilises 

 with spermatozoa. A slightly more advanced stage is seen in the frog. 

 Fertilisation is still outside the body of the mother, but the male, embracing 

 the female, liberates spermatozoa upon the eggs, which are at the same time 

 laid. 



In the majority of cases, however, special organs for emitting and for 

 receiving spermatozoa are developed, and copulation occurs. The male 

 organ is often an adaptation of some structure already existing, as in many 

 crustaceans, where modified appendages form external canals for the seminal 

 fluid. In skates and other gristly fishes, the remarkably complex copula- 

 tory organs, so-called "claspers," are in close connection with the hind 

 limb. The penis of higher vertebrates is virtually a new organ. The 

 copulation may be quite external, as in crustaceans, where the male seizing 

 the female deposits spermatozoa upon the already laid eggs. Oftener, how- 

 ever, it is internal, and the intromittent organ is inserted into the genital 

 aperture of the female. True copulation may occur without the presence 

 of special organs, — notably in the case of many birds, where the cloaca of the 

 male is apposed to that of the female. The spermatozoa, forcibly expelled 

 by the excited male organs, pass up the female ducts, probably, in part, as 

 the result of peristalsis, but chiefly at least by their own locomotor energy, 

 and one of them may eventually fertilise an ovum. In addition to the 

 intromittent organ, and the lower portion of the female duct which receives 

 it during copulation, there may be auxiliary structures, such as true 

 claspers for retaining hold of the females. The limy "cupid's dart" or 

 " spiculum amoris " of the snail, is usually interpreted as a preliminary 

 excitant. 



Three further notes in regard to higher animals are requisite, (i.) There 

 is much reason to believe that the follicles tend to burst towards the 

 end of menstruation ; that this may be accelerated by copulation ; success- 

 ful fertilisation may occur at any period, but most frequently soon after 

 menstruation, and most rarely during the relatively infertile period most 

 distant from that process. (2.) After conception, when the fertilised egg 

 has begun to develop, the mouth of the uterus is closed by a secretion, 

 which prevents the entrance of other spermatozoa should further copula- 

 tion occur. (3.) The period of gestation, i.e., between the fertilisation of 

 the ovum and the extrusion of the fcetus, varies widely in mammals, from 

 about 18 days in opossum, or 30 in rabbit, to about 280 days in Homo or 

 600 in the elephant, being longer in the more highly evolved types. But 

 it also depends on size, being about 280 days in cow and 150 in sheep ; on 

 number of offspring, being about 350 in mare and 60 in dog ; and on the 

 degree of maturity at birth, being 420 in giraffe and 40 in kangaroo. 



§ 4. Parturition. — In many cases, e.g., marine annelids, 

 mature ova burst, as we have already noted, from the mother 

 animal, who may thenceforth have nothing more to do with 

 them. Liberation of ova from the ovary and from the organism 

 may be almost coincident, as in most bony fishes. In other 



