70 FUNDAMENTALS OF CYTOLOGY 



that a wave of local stretching, or increase in area, begins at the polar 

 regions and progresses toward the site of the furrow. This stretching 

 continues as the furrow develops, the surface actually moving inward 

 along the walls of the furrow and becoming the membranes of the daugh- 

 ter cells. In at least one instance it has been reported that this ingrowth 

 stops short of the middle of the egg, the membranes in the central area 

 being formed anew in association with a granular substance accumulated 

 there. The physicochemical problem of accounting for these changes 

 in terms of alterations in viscosity, surface tension, and other factors 

 has long occupied the attention of biologists, and it is believed that 

 substantial progress is being made toward its solution. 



Highly interesting contributions to the eventual solution of this 

 problem have bepn afforded by nucleate and nonnucleate egg fragments 

 obtained from normal eggs by shaking, centrifugation, or constriction 

 with a hair noose. When nonnucleate fragments of Arbacia eggs were 

 treated with parthenogenetic agents (hypertonic sea water, ultraviolet 

 light), asters appeared and cytokinesis frequently followed. In some 

 cases numerous successive divisions occurred, giving nonnucleate blas- 

 tulae, one of which had as many as 500 cells and lived a month or more. 

 In an amphibian {Triton), similar phenomena were observed. A frag- 

 ment with a nucleus and accessory asters formed a blastula containing 

 nuclei and asters in some cells but only asters in others. Such accessory 

 asters may be retentions from previous mitoses, or they may be cytasters, 

 which have long been known to form anew in cells under experimental 

 treatment. Such facts indicate a degree of independence between the 

 nuclear cycle and the astral cycle, the two being well correlated in normal 

 cells, although either is capable of continuing alone for a limited time. 



Cytokinesis by furrowing in tissue cells presents an even more difficult 

 problem than that in free cells. The activities of any one of the cells are 

 determined in part by its neighbors, yet it seems likel.y that the principal 

 forces at work in cleaving eggs are paralleled in tissue cells. Asters may 

 be less prominent in the latter, but such cells can still have localized 

 regions of high viscositj^, and the furrows in the two cases are often 

 strikingly similar. There is also the further problem of accounting for 

 the furrows that cleave large multinucleate plasmodia into cells with 

 single nuclei. 



Between the limiting membranes of adjacent cells of a tissue is a layer 

 of material known as intercellular substance. This is primaril}^ a secretion 

 of the protoplasm and varies greatly in amount in different tissues. 

 Its physicochemical complexity is indicated by the variety of fibrous and 

 other modifications that may appear within it, and these may have a 

 large share in determining the functional value of some tissues, such as 

 certain connective tissues and cartilage. 



