terior end of the pharynx. In terms of the gut, there is no 

 room for a more anterior arch. 



On the basis of its development, the arterial system of the 

 cyclostome fish is quite modified. It does not appear to rep- 

 resent a stage from which the gnathostome system was 

 derived. Again as in some other features, the cyclostome 

 appears to be a parallel development to the gnathostome. 



The general agreement of aortic arches among the gnatho- 

 stomes does support the supposition of common ancestry. 

 The exceptions to this picture are some of the sharks where 

 additional posterior arches develop. The increase in the 

 number of arches reflects the number of pharyngeal pouches. 

 It is generally assumed that pouches may be added poste- 

 riorly by a simple process of serial replication just as the 

 number of vertebrae, or any of the serial homologs, can be 

 altered. 



The venous system shows parallel tendencies in the several 

 groups of vertebrates. Again the cyclostomes have apparently 

 solved circulation problems in a fashion distinct from the 

 gnathostomes. The great differences between the lamprey 

 and hagfish — diffisrences that are perhaps greater than the 

 extremes of the gnathostomes — suggest their origin from 

 quite divergent stocks followed by convergent evolution 

 which has produced end forms that are not particularly dif- 

 ferent in life habit and appearance. 



Among the gnathostomes, there appears to be a general 

 tendency toward the development of a postcaval stem and 

 for the establishment of a renal portal system. The conver- 

 sion of the kidney from an arterial-renal portal to a purely 

 arterial supply in the mammal is probably related to the 

 form of the metanephric kidney. The pattern of circulatory 

 change associated with kidney change appears rather early 

 and permits the view that the reptiles could have had 

 several parallel evolutionary lines, one of which gave rise to 

 the mammal. In terms of the whole circulatory system, the 

 mammal is not far removed from the general reptile pat- 

 tern, nor for that matter is any tetrapod or even gnatho- 

 stome. 



The yolk-sac circulation of the various fishes and tetra- 

 pods is of interest. The subintestinal vein is a functional prod- 

 uct of the movement of blood from the posterior end of the 

 animal — from the caudal vein out on the yolk sac and into 

 the terminal sinus, then forward through this sinus to the 

 vitelline veins, or ring sinus, and the heart. This venous 

 yolk-sac circulation is probably the basal type for verte- 

 brates. In the shark the ring sinus becomes arterial, and the 

 blood flow in the yolk sac is reversed. In the amniotes the 

 yolk-sac channels receive arterial blood from the dorsal 

 aorta through segmental (body wall) arteries situated far 

 back in the body (region of posterior limb). 



Here, as elsewhere, there is some question as to the use of 

 terms. For example, the cardinal veins of the embryo are 

 shown to be the precursors of a complex and frequently 

 subdivided drainage. Should the whole drainage be de- 

 scribed by this term or should it be restricted to a specific 

 channel within the drainage? Restrictive terms would make 



discussion extremely difficult and perhaps meaningless. Cer- 

 tainly it is reasonable to associate the terms anterior and 

 posterior cardinals with the everchanging embryonic chan- 

 nels or the similar channels in the adult. However, one can- 

 not effectively discuss internal and external jugulars as the 

 anterior cardinal even though the former is a fairly direct 

 derivative. A certain amount of judgment appears to be 

 necessary. 



BLOOD 



Comparative hematology 



Blood, a liquid tissue, circulates through the channels al- 

 ready described. There are two aspects of blood cells which 

 can be examined: first the various types and secondly their 

 sites of origin in the embryo and the adult. 



The comparative anatomy of blood cells has not been 

 thoroughly investigated. As a generality, there are four 

 types of cells present: erythrocytes, granulocytes, agranulo- 

 cytes, and thrombocytes. The erythrocytes or red corpuscles 

 are usually flattened, ovoid cells. They contain hemoglobin 

 which acts in the transport of oxygen. In the mammal the 

 erythrocytes are round, biconcava, and enucleate (without 

 a nucleus). In the camels they are enucleate but ovoid. In a 

 few frogs and salamanders, some erythrocytes fragment, 

 producing anucleate pieces which continue to function for a 

 time. Erythrocytes are characteristic of vertebrates and are 

 not found even in Amphioxus. They vary in different verte- 

 brates mainly in size. 



The white cells or leukocytes are of two types, with and 

 without granules in the cytoplasm — the nuclei of most blood 

 cells are granular. Mammahan granulocytes are of three 

 kinds based on the staining of their granules: neutral (neu- 

 trophile or heterophile), acid (eosinophile), or basic (baso- 

 phile. These are usually polymorphonucleate, that is, having 

 irregularly lobed nuclei. The agranulocytes have compact 

 nuclei, a homogeneous cytoplasm, and stain basically. 

 There are two types, lymphocytes and monocytes, differ- 

 ing in size, shape, and function; the former frequently show 

 pseudopodial extensions. 



The agranular leukocytes are not far removed from con- 

 nective tissue cells of the reticuloendothelial system, a sys- 

 tem which includes all phagocytic cells except those of the 

 blood. Phagocytosis refers to the engulfing and digestion of 

 damaged cells or bacteria. The agranular leukocytes are 

 particularly like the amoeboid (moving around like an 

 amoeba) macrophages of connective tissues. It is thought 

 that these leukocytes can become fixed in position and 

 changed in form for tissue repair or modification. 



The leukocytes of other vertebrates cannot be directly 

 compared with those of the mammal. For example, frogs 

 have a polymorphonuclear cell (with a nucleus irregularly 

 lobate) resembling the mammalian neutrophile, but with- 

 out granules. 



Thrombocytes are associated with clotting. In most verte- 



BLOOD • 377 



