THE PROTOPLASMIC SYSTEM 



terus, after it has been centrifuged with force sufficient to 

 stratify its inclusions according to the specific gravity of 

 each: between the zone of oil-drops, the lightest constituents 

 of the cytoplasm, and the yolk-spheres massed at the 

 opposite pole, lies a clear zone which under high power of the 

 microscope and with bright-field illumination appears 

 optically empty; with the dark-field or after proper fixation 

 this zone appears to be made up of extremely fine granules. 

 This clear area if cut away, even though it be but a small 

 fraction of the egg, will develop if fertilized with almost the 

 same pattern and tempo of development as the whole egg. 

 This is likewise true of eggs of sea-urchins. The conclusion 

 is that of the extra-nuclear substances the only part abso- 

 lutely necessary for life is the clear inclusion-free men- 

 struum. I therefore define as cytoplasm this menstruum 

 only. In the following pages I shall adhere stricth' to this 

 definition. 



Despite the fact that the cytoplasmic inclusions are not 

 essential to cytoplasm, they have undoubted functions and 

 merit here brief notice. In the following treatment, I speak 

 mostly of egg-cells. 



The lipins (fats and fatty substances), discussed above, 

 are present in eggs largely as oil and in yolk (lecithin). 

 The oil (fluid fat) in eggs exists as drops in various degrees 

 of emulsification. Whilst it is easy to observe oil drops in 

 some eggs in which they are relatively large — as in 

 the egg of the sea-bass, — it is often difficult to observe them 

 in some other eggs where they are in a state of fine division. 

 In the unfertilized egg of Nereis one can count the twenty to 

 twenty-two oil drops present and can as development 

 ensues after fertilization follow the gradual reduction by 

 coalescence of this number to four. In the living larval 

 worm one can further watch the emulsification of these into 

 finer and finer drops which eventually disappear — as further 

 shown by micro-chemical tests on both living and killed 



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