66 EVOLUTION AND GENETICS 



is isolated with the right ventricle, while the remainder carries 

 blood from the left ventricle into the other arches. Of these the 

 fifth pair is never large and soon disappears, along with the first 

 and second. The third pair, and its connections with the first and 

 second, persists to carry blood to the head, while the fourth differs 

 in development in the birds and mammals, but develops into the 

 great aorta in each class. The approximation of the adult struc- 

 tures of lower forms is not limited to these parts of the circulatory 

 system, for the veins also show a gradual transition, but the heart 

 and aortic arches are no less striking, and are more easily under- 

 stood. 



The excretory system develops in vertebrates from paired 

 mesodermal masses, called the nephrotomes, extending from the 

 cervical to the caudal region. In its primitive form it consists of a 

 series of tubules opening into the coelom. By ciliary action these 

 tubules carry wastes from the coelom to the exterior. The pres- 

 ence of a knot of arteries, or glomerulus, projecting into the coelom 

 near each tubule suggests an association with the circulatory 

 system (Fig. 44). Such structures as these arise from the anterior 

 part of the nephrotomes, and make up a pair of bodies called the 

 pronephroi, or anterior kidneys. The association of the excretory 

 tubules with the coelom is soon supplanted by an intimate asso- 

 ciation with the glomeruli, so that wastes are removed directly 

 from the blood (Fig. 44B). A second pair of kidneys made up of 

 such structures, arise behind the first, and are called the meso- 

 nephroi. These become the functional kidneys of adult fishes and 

 amphibia, while the pronephroi occur only in embryos and larvae. 

 At a later stage of development the mesonephroi are supplanted 

 by a third pair of excretory organs, the metanephroi. These are 

 found only in reptiles, birds and mammals, which develop during 

 their ontogeny first rudimentary pronephroi, then functional 

 kidneys of the embryo, mesonephroi, and finally the metanephroi 

 which are to persist throughout life. 



Nothing short of a thorough study of embryology can ade- 

 quately disclose the marvelous resemblances which occur one after 

 the other as the embryos of vertebrates pass through the successive 

 stages of development. In the nervous system, the respiratory 

 system, the development of the pharynx, and in countless other 

 details of structure involving various systems these indications of 

 relationship of the several classes are present. In presenting this 



