BODY CAVITY— VASCULAR SYSTEM. 



453 



ay/. 



Body Cavity. 



In Ainphioxus a paired pouch grows out from the archen- 

 teron. Ahnost at once this becomes divided up on either 

 side to form a series of small sacs, the cavities of which 

 form ultimately the true body cavity or ccelome. According 

 to Hertwig, this is in type, the method of formation of the 

 coelome throughout the Vertebrata. In the other Verte- 

 brates, owing to modified processes of development, probably 

 first arising from the presence of much yolk, we have solid 

 cell masses growing out in place of hollow sacs, but the 



cavities which appear 

 later, apparently by split- 

 ting of the cell mass, are 

 in reality the retarded 

 cavities of true gut 

 pouches. 



Vascular System. 



From Cyclostomata on- 

 wards the blood fluid 

 contains red corpuscles, 

 i.e., cells coloured with 

 haemoglobin — a pigment 

 which readily forms a 

 loose union with oxygen, 

 and bears this from the 

 exterior (gills or lungs) 

 to the tissues. These 

 pigmented cells are usu- 

 ally oval and nucleated. 

 In all Mammals except 

 Camelidfe they are cir- 

 cular. Moreover, the full 

 grown red corpuscles of 

 Mammals have no visible 

 nuclei. The blood fluid 

 also contains uncoloured 

 nucleated amoeboid cells, the white corpuscles or leuco- 

 cytes, of much physiological importance, e.g., by bearing 

 food particles from one part of the body to another. 



Fu;. 145. — Transverse section 

 through a Teleostean Embryo 

 (diagrammatic). (After Zif,gler.) 



S.C., Spinal cord; N.^ notochord; ao., aorta; 

 C.V., cardinal veins (united); s.d., segmental 

 duct; C.J ccelome or pleuro-peritoneal cavity; 

 v.v.f position of median vitelline vein; y., 

 yolk; £n., Endoderm ofgut ; ^.(/., segmental 

 duct; ;///., myotome. The dots represent 

 mesenchyme cells, the little circles blood 

 corpuscles. 



