A DEFINITION OF EVOLUTION 



ceans simply means that Cijpris and barnacles represent a side branch of 

 crustacean evolution. 



Examples Among Vertebrates. The ontogeny of man, considered bio- 

 genetically, indicates a long, complicated history. The fertilized egg cor- 

 responds to a protozoan ancestor, but it soon becomes multicellular, 

 indicating a primitive metazoan grade. Gastrulation forms a coelenterate- 

 like embryo, but this soon becomes triploblastic, like a Hat worm. Funda- 

 mental chordate characters (dorsal nerve tube, notochord, and pharvnx 

 specialized for respiration) are then developed. Fish-like characters, such 

 as gill slits and aortic arches, appear, followed by tetrapod characters, 

 such as the pentadactyl limb and metanephric kidney. Finally mammalian, 

 then primate, and at last specifically human characters appear (Figure 

 20). 



The details of the development of specific systems are sometimes very 

 impressive. The kidneys of vertebrates are all developed from the ncphw- 

 tome, a segmented mass of mesoderm lying on either side of the somites, 

 from which so many of the serially homologous structures of the body are 

 developed. Yet there are three distinct types of kidneys among the verte- 

 brates. All vertebrate embryos first develop a pronephric type of kidney, 

 utilizing onlv the anterior-most part of the nephrotome. Only in the hag- 

 fishes and a few of the bony fishes (and here only in part) does this re- 

 main as the functional kidney of the adult. In all other vertebrates 

 (including all of the bony fishes), a mesonephric type of kidney is devel- 

 oped posterior to the pronephros, and the pronephros either degenerates 

 or is partly incorporated into the mesonephros (with the exception noted 

 above). This mesonephros is the functional kidney of adult fishes and 

 amphibians, and of the embryos of reptiles, birds, and mammals. But in 

 all of the last mentioned classes, a third type of kidney, the metanephros, 

 is formed posterior to the mesonephros, and serves as the functional kid- 

 ney of the adult organism. 



Similarlv, all vertebrate embrvos develop a scries ( most commonly six ) 

 of aortic arches, each of which runs unbroken from the ventral aorta to 

 the dorsal aorta, much as in adult Amphioxus (Figure 21). In the fishes, 

 these arches are modified in several ways, all of which involve the sepa- 

 ration of each aortic arch into a ventral afterent branchial artery and a 

 dorsal efferent liranchial artery, the two being connected by a capillary 

 network in the gill filaments. In the Choanichthyes, the group of fishes 

 most closely related to the Amphibia, the first arch drops out, and is 

 largely missing in the adult, but its ventral and dorsal roots, together with 

 new growths from them, form the major arteries of the head (the external 

 and internal carotids). The sixth arch has given rise to a puhuonanj 

 branch which supjilies the lungs. This teudencv for parts to drop out after 

 having been h)niied in the embrN'o, and for the remaining parts to be 

 diverted to completely dillerent functions from the original purely res- 

 piratory function, is the principal factor in the embryology of this part of 

 tlie eireiilatory system of all tetrapods. Among the urodeles, the main 

 [)()rti()ns ot the first and second arches drop out, so that now the carotids 

 arise from the third arch. Tlu> third arch is broken b\' a eapillarv network 



48 



