63 1 



Comparative Morphology of Chordates 



are interpolated along the course of larger lymph-vessels whose fluid 

 circulates through the spaces of the spongy lymphoid tissue (Fig. 181). 

 The glands not only are seats of formation of lymphocytes but also 

 serve a highly important protective function in that phagocytic cells 

 in the endothelial walls of the lymph-spaces may remove bacteria or 

 other foreign bodies or harmful substances from the lymph-stream. 



Mammalian blood has one feature which is characteristic of the 

 Class. The fully mature erythrocytes contain no nuclei. The 



nucleus of the young red blood-cell eventually degenerates and is 

 completely obliterated. 



III. Alimentary System 



In the alimentary system the features which are most character- 

 istic of Mammalia are found in the mouth. 



Teeth 



The heterodont teeth are, in most orders, highly specialized in 

 adaptation to the nature of the food and the manner in which the 

 teeth are used. The toothed cetaceans (sperm whales and some others, 

 porpoises, dolphins) are exceptional in being homodont, all their teeth 

 having the same simple and usually conical form. In their form, cetacean 

 teeth resemble teeth of the type commonly found in fishes, but they 



Afferent lymphatic vessels 



Lymph sinus 



Capsule 



phatic vessels 



Fig. 481. Diagram representing two stages in the development of a lymph- 

 gland. The left half of the diagram is an earlier stage than the right half. (Modified 

 from Bremer: "Text-Book of Histology," Philadelphia, The Blakiston Company.) 



