GENERAL HISTOLOGY. IO/ 



the disk a slight swelling; if, however, the nucleus degen- 

 erates, the bilateral convexity is replaced by a shallow con- 

 cavity. In the latter case, one has, in reality, no right 

 longer to speak of blood-cells, since all the characteristic 

 constituents of the cell, nucleus and protoplasm, have dis- 

 appeared. Systematically the red blood-corpuscles are of 

 interest, inasmuch as unnucleated forms are found only 

 in the mammals (Fig. 44, a, b), nucleated ones in all the 

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

 the other vertebrates oval, disks. In this latter respect, 

 however, exceptions occur, since among the mammals the 

 Typloda (Camel, Llama) have oval, among the fishes the 

 Cyclostomes have circular, blood-corpuscles. 



Haemoglobin. The red blood-corpuscles are the cause 

 of the color of the blood, as well as the supporters of one 

 of its most important functions, the performance of the 

 interchange of gases; both are connected with the fact 

 that the stroma contains the coloring matter of the blood 

 or haemoglobin. Haemoglobin belongs to the less crystal- 

 lizable proteids and is remarkable for the presence of the 

 extremely important, though small, quantity of iron, and 

 also for its affinity for oxygen. Haemoglobin containing 

 oxygen, oxy-haemoglobin, causes the carmine-like color 

 of the so-called arterial blood ; oxygen-free, " reduced ' 

 haemoglobin causes the dark red, faintly bluish color of 

 the 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 white blood-cells, which on that account are called 

 lymph-corpuscles. 



In the majority of invertebrated animals there is 

 present only one kind of nutritive fluid, and not even this 

 in every class; the fluid is called blood, although it is 

 usually colorless. Where color is present, it is generally, 

 if not always, a yellowish red or an intense red ; this may, 

 even as in the vertebrates, be caused by haemoglobin 

 (among the mollusks in Planorbis, Area tctragona, A. Note, 



