4 8 4 



STRUCTURE OF VERTEBRATA. 



sc 



from the archenteron (enterocoelic). In the other Vertebrates, 

 owing to modified processes of development, probably first 

 arising from the presence of much yolk, solid cell masses 

 grow out in place of hollow sacs, but the cavities which 

 appear later, apparently by splitting of the cell mass 

 (schizoc<xlic\ are in reality the retarded cavities of true 

 ccelom-pouches. In the adult it is divided into an anterior 

 pericardial and a posterior peritoneal portion. 



The body cavity may form part 

 of one or all of the following 

 systems: (i) excretory, void- 

 ing waste by abdominal pores or 

 by nephrostomes ; (2) reproduc- 

 tive, receiving the liberated 

 genital elements ; and (3) lym- 

 phatic, receiving tiansudations 

 from visceral and abdominal 

 organs. 



It is probably never quite 

 closed, but may communicate 

 with the exterior by abdominal 

 pores (or through nephrostomes) 

 opening into the renal system. 

 Both occur together in some 

 Elasmobranchs, but they are 

 usually mutually exclusive. In 

 the higher Teleostei, in some 

 Saurians, and in Mammals, 

 there are neither abdominal 

 pores nor nephrostomes, but 

 FIG. 233. Transverse section through only openings (stomata) into the 



a Teleostean embryo (diagram- lymp ] iat i c sys tem. 



matic). After Ziegler. 



ml 



s.c., Spinal cord; M, .notochord ; ao 

 aorta; c.v., cardinal veins (united); s.a., 

 segmental duct; c., ccelom or pleuro- 

 peritoneal cavity; v.v., position of 

 median vitelline vein ; y., yolk ; ., 

 endoderm of gut ; int., myotome. The 

 dots represent mesenchyme cells; the 

 little circles, blood corpuscles. 



VaSCUlai System. - 

 rom CyclOStOmata 



J ^ hlnorl flnirl /-rm 

 WardS ttlC Fluid CO1V 



tainS red COrDUScleS, i.e. 

 . . x . , , 



CellS Coloured With haemO- 



globin a pigment which 

 readily forms a loose union with oxygen, and bears it from 

 the exterior (through gills or lungs) to the tissues. These 

 pigmented cells are usually oval and nucleated. In all 

 Mammals except Camelidae they are circular. Moreover, 

 the full-grown red corpuscles of Mammals have no visible 

 nuclei. The blood fluid also contains uncoloured nucleated 



