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THE AMEBIC AN NATURALIST [Vol. XLII 



egg. Normal fertilization is supposed to act in the same way. 

 The evidence at hand does not appear to the reviewer to sub- 

 stantiate this generalization, because the shrinkage sometimes 

 observed in the normal egg may be an effect of changes taking 

 place rather than their cause, and because, in the second place, 

 some of the most recent results, especially those with weak acids 

 (Lefevre),can scarcely be interpreted on Przibram's view. There 

 are some indications, on the contrary, that fertilization is a vital 

 act in the sense that the sperm is a stimulus starting develop- 

 ment in the same way thai many external agents call forth the 

 nerve impulse in a nerve. Here also in the nerve the same im- 

 pulse may be called forth either by internal factors or by ex- 

 ternal agents. The intimate nature of irritability of living 

 material remains, however, as much of a puzzle to the physiolo- 

 gist as to the embryologist. 



The occurrence of visible inclusions in the egg, — pigment, 

 yolk, granules, etc.. — leads the author to conclude that even be- 

 fore fertilization the egg is made up of different substances, 

 which "guarantee" the subsequent development. We touch 

 here on one of the burning questions of modern embryological 

 speculation, for while certain evidence seems to show that dif- 

 ferent substances contained in the egg furnish a "guarantee" 

 of diversity, still the central problem remains untouched, for 

 few, if any. experimentalists will admit unreservedly that these 

 materials are preformed primordia. or organ-forming substances 

 in a strict sense, rather than that their presence is a condition 

 that prejudices in certain directions the subsequent development. 

 In other words we are still unable to state how far and in what 

 sense the development is due to preformed materials and how 

 far it is an epigenetic phenomenon depending on the relation 

 of the parts to each other in a dynamic sense. It may be well 

 in the present unsettled state of the subject not to prejudice 

 these questions, for, there may be truth on both sides, since pre- 

 determination and epigenesis are not mutually exclusive possi- 



In regard to the location of the first cleavage plane. Przibram 

 points out that this is "given" by the location of the segmenta- 

 tion spindle. The position of the spindle itself is the result of 

 the egg structure, the geometrical form of the egg and the 

 meridian of fertilization. The position of the spindle in any par- 

 ticular case may be due to any one or to more than one of these 

 factors. The causal problems still remain to be considered. 



