CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 359 



is a general and indefinite term which may include anything 

 from metaphysical entelechies to hard and fast structures. 

 Omitting from consideration all hypothetical causes which are 

 beyond the reach of experimental investigation, we find that 

 polarity, development, regulation or any other vital phenomenon 

 may be regarded from the standpoint of static or of kinetic con- 

 ditions, of morphological or of physiological causes. Ideally 

 these distinctions are sharp and definite, but they are not so in 

 reality. In a living organism static and kinetic, morphological 

 and physiological conditions are really inseparable. 



However, for the sake of clear thinking, it is necessary to form 

 some sort of a mental picture of what is meant by such a phrase 

 as 'the organization of the egg.' On its morphological side the 

 polar organization consists, as I have attempted to show, in a 

 relatively persistent framework of viscid material which is also 

 elastic and contractile so that it tends to resume its normal form 

 when distorted; by this framework nuclei, centrospheres and 

 mitotic figures are bound more or less firmly to the peripheral 

 layer or ' Hautschicht' (Strasburger) . There is no good evidence 

 that this viscid material exists in the form of fibers which are 

 definite in number and position. On the other hand its behavior 

 during centrifuging would indicate that the appearance of fibers 

 is due to inclusions which are forced into an otherwise continu- 

 ous substance; this substance is more abundant and more uni- 

 fomily continuous at the animal pole than elsewhere in the cell. 

 In normal eggs the presence of large yolk spherules at the vegetal 

 pole gives to this substance a coarse sponge-like texture while 

 the smaller spherules of yolk, oil and enchylemma toward the 

 animal pole give to it a finer alveolar character. This appear- 

 ance is very evident in good sections of normal eggs, as is shown 

 in the figures of my paper on Karyokinesis and Cytokinesis 

 (Conklin '02), and that this structure is not an artifact, but is 

 normal, is confirmed by the experimental studies of this and of a 

 former paper (Conklin '12). 



When yolk is driven to the animal pole by centrifugal force the 

 fluid portion of the cytoplasm and much of the viscid portion are 

 driven away and that which remains is stretched or compressed 



