L12 HISTOLOGY. 



medullary plate. From here the neuroblasts wander out and 

 collect to form cell groups. The later development has been 

 worked out by His. A process grows from each pole, one cor- 

 responding with the axone and the other with the dendrites. 

 The axone grows centralward and penetrates the wall of the 

 medullary tube. Here it develops subsequently into a fibre 

 which enters into the formation of the dorsal columns. The 

 dendritic process proceeds in the opposite direction. By a sub- 

 sequent change in the shape of this bipolar cell the axone and 

 dendrites both come to be processes from one outgrowth of the 

 cell. In other words, the cell becomes unipolar (Fig. 71). 



V. BLOOD AND LYMPH. 



Blood and lymph are properly to be considered as tissues 

 consisting of formed elements in a fluid intercellular substance. 



1. Blood. 



The blood of the higher animals is a red fluid, which is 

 made up of blood plasma (intercellular substance) and formed 

 elements (blood corpuscles, blood platelets, and various gran- 

 ular elements). We distinguish two kinds of blood-corpuscles : 

 red (colored) and white (colorless). 



Red blood-corpuscles (also known as erythrocytes) contain 

 the red coloring material, hcemoglobin, which gives to them 

 and to thin layers of blood a straw-yellow tint. They are in 

 mammals almost without exception flat, round structures with- 

 out a nucleus. The flat surfaces are depressed in the centre, 

 giving to the cell the general form of a biconcave lens. The 

 borders are rounded and much thicker than the centre. In 

 optical cross-section the corpuscle is biscuit-shaped. 



The red corpuscles vary in size in different animals, from 

 2.5 11 (in Moschus javanicus) to 9.4 (j. (Elephas indicus) in diam- 

 eter. In man they are from 7.2 to 7.8^ in diameter, and 1.9^ 

 thick at the thin middle point. Oval red corpuscles are found 

 among mammals only in the llama and the camel. They are, 

 however, common in lower animals. The red blood-cells of 

 ii-hes, amphibians, reptiles, and birds, are oval in form and 



