GENERAL HISTOLOGY 



79 



Red Blood-corpuscles. In the mature condition the red blood-corpuscles 



of vertebrates (fig. 47) are circular or oval discs, which by external influences 

 (e.g., pressure) may temporarily be bent or otherwise modified in form, but 

 cannot actively change their shape, because they no longer consist of protoplasm. 

 Embryologically they arise from true, nucleated, protoplasmic cells; gradually 

 the protoplasmic cell-body changes completely into a plasmic product, the 

 stroma of the blood-corpuscle. If the nucleus be retained in this metamorphosis, 

 there is a slight swelling in the centre of the disc (fig. 47, d'); if, however, the 

 nucleus degenerate, the bilateral convexity is replaced by a shallow concavity. 

 In the latter case, one has, in reality, no right to speak of blood-cells, since the 

 characteristic constituents of the cell -nucleus and protoplasm have disap- 

 peared. Systematically the red blood-corpuscles are of interest, since non- 

 nucleate forms are found only in the mammals (fig. 47, a, b), nucleated ones in 



FIG. 47. Red blood-corpuscles, a, of man; ?>, of the came!; c, of the adder; </', of 

 Proteus (seen from the edge); d", surface view; e, of a ray;/, of Petromyzon; n, nucleus 

 (all the blood-corpuscles are magnified 700 times, except d, which is magnified 350 

 times). 



all the other vertebrates (c, d}. The mammals also have circular, the other 

 vertebrates oval, discs. To this, however, exceptions occur, since among the 

 mammals the Tylopoda (camel, llama) have oval, the cyclostomes have circular 

 blood-corpuscles. Recent investigations tend to show that the corpuscles, at 

 least in mammals, have a hat shape while in the living blood-vessels and that 

 they become disc-like when the normal conditions are interfered with. 



Haemoglobin. The red blood-corpuscles cause the color of the blood, and 

 are the agents of one of its most important functions, the interchange of gases; 

 both are connected with the fact that the stroma contains the coloring matter, 

 hemoglobin, of the blood. Haemoglobin is one of the few crystallizable proteids 

 and is remarkable for the presence of a small, though extremely important, 

 quantity of iron, and also for its affinity for oxygen. Haemoglobin containing 

 oxygen, oxy-h&moglobin, causes the carmine-like color of the so-called arterial 

 blood; oxygen-free, 'reduced' haemoglobin causes the dark bluish-red color of 

 venous blood. 



Lymph is distinguished from blood by the entire lack of red blood-corpuscles 

 and the slight coagulability of its plasma. Lymph is accordingly a proteid- 

 containing fluid with leucocytes, which are here called lymph-corpuscles. 



In the majority of invertebrates there is only one kind of nutritive fluid, and 

 not even this in every class; the fluid is called blood, although it is usually color- 

 less. Where color is present, it is generally, if not always, a yellowish-red or 

 an intense red; this may, as in the vertebrates, be caused by haemoglobin (some 

 molluscs, annelids, and insects). Often other coloring matter occurs instead of 

 haemoglobin: in the cuttlefish, many snails, and in the lobster and Limulus, the 



