REPRODUCTION. 901 



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. Regard- 

 ing the former little is known. Regarding the latter, recent assertions of the 

 general invigorating effects of injections of testicular extracts in the adult, 

 although in most cases not founded upon careful experimentation, are, never- 

 theless, suggestive, and 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, and 

 in the case of the thymus and thyroid glands its occurrence seems undoubted 

 (p. 205). As to the reproductive organs, investigation of the subject is yet in 

 its mere infancy, and it is too early to say with any degree of authority what 

 the truth of the matter is. Very recently Zoth l has shown that daily injec- 

 tions of testicular extract during one week increased by 50 per cent, the work- 

 ing power of a man's neuro-muscular system. The increase manifested itself 

 both by lessened susceptibility to fatigue and, in a still higher degree dur- 

 ing the periods of rest from labor, by increased power of recovery. What 

 part of the whole neuro-muscular system is affected by the specific substance 

 is not decided. 



D. THE REPRODUCTIVE PROCESS. 



Attention has heretofore 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. 2 



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, 



1 O. Zoth : Pfliiger's Archivfur die gesammte. Physiologic, Ixii., 1896. 



2 Compare Th. Boveri : " Befruchtung," Merkel und Bonnet's Eryebnisse der Anatomic und 

 Entwickelungsgeschichte, i., 1892. 



