102 EVOLUTION AND GENETICS 



It is certain that the gonads, the organs in which they are devel- 

 oped, originate from that layer. The gonads are commonly 

 formed in the wall of the body cavity, and the gametes discharged 

 into that cavity. Openings in some animals connect the coclom 

 with the exterior apparently solely for the escape of the germ cells, 

 but the excretory tubules or nephridia with the same relations 

 provide another convenient means of egress, which results in a 

 common association of reproductive and excretory systems. 



The formation of two types of gametes, the ova and spermatozoa, 

 demands different provisions for their escape and union, but the 

 association of nephridia and gonads remains obvious throughout 

 the higher phyla. 



Reproduction of Vertebrates. In the vertebrate classes the 

 transition of reproductive functions illustrates first in the cyclo- 

 stomes, fishes and amphibia the simpler stages of sexual repro- 

 duction. The gametes of most species are discharged from the 

 body and united in the water, where reproduction occurs in all 

 three classes, with a very few exceptions among. the amphibia. 

 Terrestrial reproduction in the Sauropsida, including the reptiles 

 and birds, demands additional modifications which are chiefly 

 expressed in the foetal membranes treated under embryological 

 relationship. Internal fertilization of the ovum is, however, a 

 functional modification of equal importance, since the fluid medium 

 is essential for the union of the germ cells. 



The addition to the ovum in oviparous species of enough food 

 to carry the young animal through its development to a point 

 where it is capable of functioning as a more or less independent 

 terrestrial organism is accomplished by the assumption of the 

 function of secreting these substances by the tissues associated 

 with the passage of the egg from the body. 



The structural and functional modifications associated with 

 the development of large eggs containing much food are not 

 changed in the lowest mammals, but here a new function, with 

 accompanying modification of structures, makes its appearance; 

 viz., the secretion of milk for the nourishment of the young. These 

 mammals are the lowly oviparous Prototheria, the duck mole and 

 spiny anteater of Australia. The next division of the mammals, 

 the Metatheria, includes such species as the kangaroos and opos- 

 sums. In these the highly developed ducts of the female gonads 

 assume a new function, that of providing nourishment through 



