THE VASCULAR SYSTEM 



365 



specialized blood cells occur both in pre-chordates and chordates. The 

 lymphocytes of cyclostomes resemble those of some annelids. The 

 erythrocytes of gephyreans are similar to those of vertebrates and some 

 echiuroids have enucleated red blood corpuscles Hke those of mammals. 



The primordial blood cell of animals appears to have been a lymphocyte 

 from which the various types of blood cells— erythrocytes, granulocytes, 

 monocytes, etc.— are differentiated. The original lymphocytes in 

 vertebrate embryos are derived from the splanchnic mesoderm. All 

 blood cells of vertebrates are therefore believed to be mesodermal. That 



iHIND GUT 



•ALiAN-ro I S 

 t PANCREAS 



SMALL INT. 



•',' / ^vjCAECUM 



ECTUM 



lUROGENlTAL 

 SINUS 



'ALLANTOIS 



Fig. 304. — Diagrams of early stages of development of the human embryo. A is an 

 early human embryo, B an embryo of four weeks and C one of five weeks. Blood cells 

 make their first appearance in the wall of the yolk-sac. Since the yolk-sac is to be 

 regarded as a herniated portion of the alimentary canal, the conclusion is drawn that in 

 the phylogenesis of the vascular system blood made its first appearance in the wall of the 

 alimentary canal. (Redrawn after Mall and Romer.) 



the first blood cells of vertebrate embryos appear in the wall of the yolk-sac 

 is not surprising since the yolk-sac is a herniated portion of the alimentary 

 canal and its mesodermal wall therefore splanchnic. 



The spleen is the fundamental blood-forming organ. In cyclostomes, 

 the splenic tissue lies in the submucosa of the stomach and intestine, 

 in the lamprey within the spiral valve. In the dipnoi, splenic blood- 

 forming tissue is concentrated in the stomach wall. In elasmobranchs and 

 ganoids, the spleen becomes attached to the mesentery and in amphibia 

 remains the chief erythrocytopoietic organ. In some amphibian larvae, 

 however, the mesonephros also functions as a blood-forming organ. 



With the appearance of hollow bones in the anura, the spleen begins 

 to lose its importance as an erythrocytopoietic organ. In mammals, 



