REPRODUCTION. 463 



sively await the reproductive act. The view that they are more than this is 

 supported by some, although slight, experimental evidence. Notwithstanding 

 the fact that removal of the testis or the ovary in adult life is often unaccom- 

 panied by great somatic changes, the profound effects of early castration upon 

 development, in both the male and female, show that upon the presence of the 

 sexual organs depends the appearance of many of the secondary sexual cha- 

 racters — characters which apparently are independent of those organs, and yet 

 of themselves distinguish the individual as specifically masculine or feminine. 

 The mode of dynamic reaction of the sexual organs upon the other organs can 

 at present be little more than hinted at. It is entirely probable that such 

 reaction is either nervous or chemical, or perhaps it is both combined. 

 Regarding the former little is known. Regarding the latter certain facts 

 point to a possible normal and constant contribution of specific material by 

 the reproductive glands to the blood or lymph, and thus to the whole body. 

 Such a process is spoken of as internal secretion. This subject is discussed 

 more fully in Vol. I. p. 273. 



D. The Reproductive Process. 



Thus far attention has been given to the general functions of the repro- 

 ductive organs. We come now to the special phenomena connected with the 

 reproductive process itself, and have to trace the history of the spermatozoon, 

 the ovum, and the embryo. It should be borne clearly in mind that the 

 essential part of the reproductive process is the fusion of the nuclei of the two 

 germ-cells. Investigation is making it more and more probable that the 

 spermatozoon and the ovum, although so different in appearance and general 

 behavior, are fundamentally and in origin both morphologically and physi- 

 ologically equivalent cells. In the processes of their growth and maturation 

 they are secondarily modified, the one into an active locomotive body, the other 

 into a passive nutritive body. The modifications in both are confined, how- 

 ever, to the cell-protoplasm (cytoplasm and centrosome) ; the essential parts, 

 the nuclei, remain unmodified and both morphologically and physiologically 

 equivalent down to the time of their fusion in the process of fertilization. 

 The many and complex details of the reproductive process exist for the sole 

 purpose of bringing together these two minute masses of chromatin. 1 



Copulation. — Copulation is the act of sexual union, and has for its object 

 the transference of the semen from the genital passages of the male to those of 

 the female. It is preceded by erection of the penis and turgidity of the organs 

 of the vulva. These latter occurrences are in the main vascular phenomena, 

 and are brought about by a distention of the cavernous spaces of the erectile 

 tissues with blood. The vascular phenomena are, however, accompanied by 

 complex nervous and muscular activities. As regards the penis, the arteries 

 supplying the organ relax and allow blood to flow in quantity to the corpora 

 cavernosa and the corpus spongiosum. Simultaneous relaxation of the smooth 



1 Compare Th. Boveri : " Befrnchtunp;," Merkel und Bonnet's Ergebnwe der Atiatomie und 



Entwickelungsgeschichte, 1892, i. 



