FOURTH LECTURE. 



PROTOPLASMIC MOVEMENT AS A FACTOR OF 

 DIFFERENTIATION. 



EDWIN G. CONKLIN. 



THE fundamental problems of development and inheritance 

 are in the last analysis questions of differentiation. Develop- 

 ment is progressive differentiation coordinated as to time and 

 place ; hereditary likeness consists in the repetition by the off- 

 spring, at certain stages of its life cycle, of definite differentia- 

 tions of the parent ; and hereditary unlikeness, or variation, is 

 a modification of these differentiations either as to their char- 

 acter or as to the time of their appearance. The phenomena of 

 differentiation are therefore of the greatest interest, and their 

 causes one of the most important problems of biology. 



In many respects the simplest and yet most important phe- 

 nomena of differentiation occur in the early stages of develop- 

 ment, while the later differentiations of tissues and organs are 

 more complicated and less general in character. The polarity 

 of the egg is one of the earliest differentiations of the de- 

 veloping organism ; it consists not only in the aggregation of 

 yolk at one pole and of protoplasm at the other, but also in 

 the establishment of certain structural peculiarities which in 

 most cases determine the position and direction of the two 

 maturation spindles and cause the first two cleavage planes to 

 pass through the polar axis of the egg. Further, it has a defi- 

 nite prospective significance, since it is probable that in all ani- 

 mals it determines the ectodermal and endodermal poles of the 

 embryo, while in most cases the animal and vegetal poles of 

 the egg give rise, respectively, to the apical and oral poles of the 



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