TISSUE FORMATION 299 



of a smooth, oiled cannula, in glass dishes kept at a temperature near the 

 freezing point of water, the changes which the cells undergo are much less 

 pronounced, and although under these conditions the cells still agglutinate 

 with one another, the agglutination is less firm, the cells remain preserved 

 much better and gradually sink down to the bottom of the dish, where they 

 form a connected, relatively thin layer of tissue. 



However, whether we use the first or second method, in principle we have 

 to deal with the same change in the constitution of the cells. Within the blood- 

 channels of the animal the amoebocyte represents a flat elliptic transparent 

 disc, which is carried along by the blood-lymph current and is not sticky ; but 

 under the influence of mechanical and various kinds of chemical stimuli the 

 amoebocyte seems to take up some fluid from the surrounding medium and 

 becomes a round or oval cell with larger granules which are separated by a 

 considerable amount of intergranular substance. As a result of this change in 

 consistency, especially of the outer ectoplasmic layer of the protoplasm, the 

 cells become sticky and adhere to one another as well as to the more or less 

 solid surface of the dish with which they come in contact, or they sink down ; 

 furthermore, associated with this change there is a tendency of the amoebo- 

 cytes to send out pseudopods and to manifest amoeboid movement. These 

 observations suggest that agglutination - and amoeboid movement may be 

 related processes. Conditions which tend to increase the consistency of the 

 protoplasm within a certain range, also tend to decrease the stickiness and 

 agglutinability of the cells and to diminish their amoeboid movement. Under 

 the action of these factors the pseudopods become fine, more or less shred- 

 like, and the amoeboid movement is slowed down. Such effects are produced, 

 for instance, by the use of hypertonic salt solutions, by addition of a slight 

 amount of acid to a sodium chloride solution isotonic with sea-water, by an 

 increase in certain ions, as for instance, Na and S0 4 , in the surrounding 

 medium, and by exposing the cells to cold. In a limited way, a temporary result 

 of this kind is also brought about by a relatively strongly alkaline NaCl solu- 

 tion. On the other hand, a softening of the cells increases agglutination and, 

 to a certain extent, amoeboid movement; a moderate amount of alkali in an 

 isotonic NaCl solution, hypotonic solutions, an increase in certain ions (K, 

 NH 4 , N0 3 ), and a slight rise in temperature, exert the latter effects. The 

 blood serum of Limulus and extracts of Limulus tissue act in a similar way, 

 and they likewise have a tendency to cause an extension and spreading-out of 

 the amoebocytes on the surface of a glass on which these cells rest. This 

 spreading-out is due to a softening of the cells ; it represents a modified type 

 of amoeboid movement, and furthermore, together with the processes which 

 take place during amoeboid movement, it explains the tissue-stereotropism 

 which is common to amoebocyte tissue and to mammalian epidermal and other 

 tissues. In general, all these different modes of reaction of the amoebocytes 

 correspond to variations in the consistency of the protoplasm, and such 

 variations explain the diverse structural types which the cells, singly or com- 

 bined into tissues, may assume ; in addition they explain the modifications in 

 the character of amoeboid movement which may be observed. Moreover, 



