THE CELLULAR MECHANISMS OF DEFENCE 1073 



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 many 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 avascular 

 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 phago- 

 cytic 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 remove the skin with- 

 out causing haemorrhage. The first effect noticed in the immediate neigh- 

 bourhood of the injury is a dilatation of the vessels, especially of the venules, 

 with acceleration of the blood flow. In the course of an hour the capillaries 

 also become dilated, and many capillary channels, previously invisible, are 

 now occupied with blood. Through the dilated capillaries there is a rapid 

 blood stream, the corpuscles occupying the axis of the vessel, so that there 

 is a periaxial layer of plasma. A little later this acceleration gives place 

 to a slowing of the blood stream, and simultaneously the leucocytes of the 

 blood are seen to be adherent to the capillary wall. Apparently the latter 

 becomes what we may call ' sticky/ the effect of the stickiness being to 

 increase the resistance to the passage of the blood through the vessel and 

 also to cause the adhesion of the leucocytes to the wall. As the current 

 becomes still slower, the distinction between axial and peripheral streams 

 disappears. The corpuscles are closely packed together, the white cor- 

 puscles being predominant at the margins of the capillary, where they form 

 a lining to the vessel (Fig. 493). The next stage is the emigration of the 

 leucocytes. These may be observed to thrust a process through the vessel- 

 wall (according to Arnold this process of emigration always occurs through 

 the stigmata, i. e. the points where the endothelial cells come in contact 

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