THE BIOLOGY OF THE CELL SURFACE 



to the mechanical action of the fibrin upon the implanted 

 tissue, but the spreading out of the cells into thin sheets 

 seems to result largely from the activities of the cells them- 

 selves. These activities, which are common to several 

 tissues, in fact to all except the very inert yolk-laden endo- 

 derm and, perhaps, the notochord, may be referred to a 

 form of protoplasmic movement having its seat in the 

 hyaline ectoplasm found at the angles and sometimes at the 

 borders of the cells. The movement cannot be observed 

 clearly in the larger masses of cells on account of their 

 opacity, but it may be seen very clearly in those cells which 

 leave the main masses and wander off by themselves. 

 These cells are irregular in shape, varying from unipolar 

 form and having a varying amount of ectoplasm at their 

 angles. The movement is amoeboid in character and 

 results either in a change in shape of the cells or in their 

 movement as a whole. Such cells are found usually in 

 greatest numbers in preparations of the medullary cord, and 

 from the cranial ganglia (branchial ectoderm), that gives 

 rise by its movement to long fibres. Cells of the epidermis 

 show their power of movement in somewhat different form. 

 As has frequently been observed, the general tendency of 

 isolated bits of epidermis is to round off into small vesicles, 

 which, when left in water, may move about for days by 

 means of their cilia. Within the lymph the same thing 

 frequently takes place, although there is apparently 

 greater resistance to the process of rolling up, and the 

 cells may often remain together in the form of extensive 

 sheets. Along the free border of these sheets of cells there 

 often appears a fringe hyaline of protoplasm, which under- 

 goes continuous amoeboid changes. In one case of this 

 kind it was observed that the sheet of cells gradually 

 spread out toward the side on which this fringe was placed. 

 Since the work of Peters (1885— 1889) it has been generally 

 admitted that wound healing in the epidermis is primarily 

 due to the movement, in part amoeboid, of the epithelial 

 cells, so that it seems quite possible that in this fringe of 

 hyaline protoplasm above described, we have one part of 

 the mechanism by which the movement of cells in wound- 

 healing is brought about. The most inert of all the tissues 

 is the endoderm, which will remain for days in the lymph, 

 practically unchanged, gorged with yolk and devoid of hya- 

 line ectoplasm. The notochord is also very inactive, 

 although large pieces of this structure may show after a 



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