1146 PHYSIOLOGY 



to the death of the host. The same process of phagocytosis may be 

 studied in its simple form, by injuring or infecting some tissue which 

 is free from blood-vessels. Thus the tail fin of an embryonic axolotl 

 may be cauterised with silver nitrate, or a small quantity of fluid con- 

 taining carmine granules may be introduced by means of a hypo- 

 dermic syringe. In either way a certain number of cells are destroyed 

 and the dead tissue thereupon acts as a foreign body. As a result 

 the wandering mesoderm cells or leucocytes move from the surround- 

 ing tissues towards the seat of the injury, and the day after the injury 

 has been inflicted a collection of leucocytes can be seen, many of which 

 contain particles of carmine or debris of the destroyed tissue which 

 they have taken up. The cells finally wander away from the part, 

 and the destruction is made good by the proliferation of the connec- 

 tive tissue-cells and of the epithelium immediately adjoining the injury. 

 In the lowest types of metazoa it is impossible to speak of more than 

 one type of wandering mesoderm cell. It is probable indeed that the 

 same type of cell may at one time act as a scavenger and at another 

 as the chief agent in the formation of connective tissues. Even in 

 Daphnia, according to Hardy, only one form of leucocyte is present, 

 whereas in the much more highly organised crayfish, belonging, 

 however, to the same family, three different types of leucocyte may 

 be distinguished. These leucocytes may be present free in the body 

 cavity or they may form, an element of the connective tissues. With 

 the formation of a closed vascular system many of the wandering 

 mesoderm cells became attached to this system, so that we may 

 distinguish a group of blood leucocytes or phagocytes and a group of 

 connective tissue or body-cavity leucocytes. Moreover by the formation 

 of a blood vascular system all the tissues of the body are brought into 

 material relationship with one another, so that distant parts may be 

 drawn upon to supply the needs of any one part. It is evident that 

 injury of a tissue in a higher animal containing blood-vessels will 

 involve more complex consequences than a similar injury or infection of 

 the a vascular tissue of an invertebrate, and that the accumulation of 

 cells for the defence of the organism against invading microbes will be 

 much more effective if the blood-vessels participate in the process so 

 that, by their means, the phagocytic resources of all parts of the body 

 can be drawn upon to ward off a localised attack. The process of 

 phagocytosis thus, in the higher animals, becomes merged into the more 

 complex series of phenomena to which the term ' inflammation ' has 

 been applied. This process can be studied by observing the effects 

 of slight injury to some transparent part of the body, e.g. the 

 frog's tongue or mesentery, or the web of the frog's foot. For 

 this purpose a small piece of the skin of the frog's web is snipped 

 off with fine curved scissors, the section being sufficiently deep to 



