248 



STRUCTURE OF THE VERTEBRATES 



by a lack of haemoglobin in each cell, or by a lessened number 

 of cells in the blood. In the normal human there is an average of 

 four and a lialf to five million erythrocytes in a cubic millimeter 

 of blood. There is a slight sex difference, the female averaging 

 less than the male. 



Erythrocytes vary markedly in structure in the different 

 vertebrate classes. With the exception of mammals all verte- 

 brates have nucleated red cells, and most of these are elliptical 

 in shape. The largest known are found in the urodele amphibia. 

 Although essentially the same shape in the reptiles and birds 

 they grow progressively smaller and more numerous. 



B. Amphibian 



C. Lizard 



CD 



D. Camel 



E. Bird 



Fig. 138. Eiythrocyte Types. Diagram (D) shows the enucleated, oval 

 erythrocyte of the camel. 



Fig. 139. Development of Mammalian Erythrocytes. Diagrams illustrating 

 the changas which occur in the marrow of the long bones; a surface view 

 of a red cell with fragmented nucleus; and a side view of an erythrocyte. 



All normal adult mammals have red cells without nuclei. These 

 cells, as in all bony vertebrates, arise in the marrow of the long 

 bones. In their early state these cells have nuclei, and are car- 

 ried into the blood stream of the embryo as nucleated cells. But 

 before birth (or shortly after in some species) the erythrocytes 

 begin losing their nuclei before entering the blood. The nucleus 

 of the maturing red cell is thrown out and disintegrates, the 



