380 HISTORY OF THE HUMAN BODY 



This absolute necessity of a fluid medium causes the de- 

 velopment in land forms of a number of accessory parts. 

 Thus there develop in the male special glands to supply a ve- 

 hicle for the spermatozoa, forming a spermatic fluid; and as 

 this cannot be allowed to dry up, it must be conveyed directly 

 to the female by an internal copulation, necessitating again 

 certain modifications of the cloacal margin, from which de- 

 velop the various external organs. Although it is evident that 

 the development of the process of copulation is here due solely 

 to the terrestrial life, there are sometimes other conditions 

 that develop it, for although, on the one hand, it is universal 

 among terrestrial forms, invertebrates as well as vertebrates, 

 it is occasionally found among aquatic animals, notably in this 

 connection the selachians; that, however, it is here an inde- 

 pendent development is shown by the source from which the 

 copulatory organs are derived, namely, from the inner margin 

 of the ventral fins, and not from the rim of the cloaca, as in 

 higher vertebrates. 



There are thus two groups of accessory reproductive organs 

 to be considered, (i) those which furnish an outlet for the 

 germ cells, and (2) those which are concerned in internal 

 copulation. These may be taken up in order. 



The conception of the peritoneal cavity as an expanded go- 

 nadic sac demands that the germ cells generated in its wall 

 should break loose and float about within the coelom, until 

 finally expelled either through some direct channel of com- 

 munication with the exterior, the original gonadic ducts, or 

 else by utilization of some part of the nephridial system ; and 

 as a matter of fact all conditions found in vertebrates, with 

 .the possible exception of that of teleosts, may be directly re- 

 ferred to one of these methods. 



The most primitive condition is that seen in cyclostomes, in 

 which the peritoneal cavity communicates directly with the 

 exterior by means of a pair of pori abdominales, canals which 

 begin at the posterior part of the ccelom and open along the 

 sides of the cloacal orifice. The germ cells, when matured, 

 become freed from' their place of origin and float about in the 



