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SCIENCE. 



[N. S. Vol. IX No. 218. 



ular body formed just outside the nucleus, 

 which enlarges by the addition of granules 

 similar to those found in the nucleus, until it 

 becomes first a cap- shaped mass and finally 

 surrounds the nucleus. 1st Period : Forma- 

 tion of the 'yolk-mass.' This body then dis- 

 integrates, the constituent granules being 

 spread evenly throughout the now highly 

 vacuolated cell-body. The latter was shown 

 to be composed probably of a pseudo-nuclein. 

 2d Period : Disintegration of the yolk-mass. The 

 ovum assumes its final character by the 

 progressive vacuolization of the cell-body, 

 and by the enlargement of the products of 

 disintegration of the 'yolk-mass' to form the 

 definite ' deutoplasm ' spheres. Sd Period : 

 The original was considered to be of nuclear 

 origin, and is probably what has been 

 loosely homologized in some cases with the 

 ' corps vitellin de Balbiani,' etc. 



Protoplasmic Movement as a Factor of Differen- 

 tiation. Edwin G. Conklus-. 

 Vaeious factors have been suggested by 

 different persons as the causes of differen- 

 tiation, but so far no one has shown that the 

 active movements of protoplasm constitute 

 such a factor. 



The polarity of the egg and the speciali- 

 zations of cleavage are two of the earliest 

 differentiations of the developing organ- 

 ism. In the gasteropod Crepidida both 

 of these differentiations are associated with 

 definite and orderly movements of the pro- 

 toplasm . 



Before the maturation the germinal ves- 

 icle lies near the center of the egg and the 

 yolk is uniformly distributed. With the 

 appearance of the centrosomes and the for- 

 mation of the first maturation spindle the 

 nuclear membrane is broken opposite the 

 poles of the spindle, nuclear sap escapes 

 into the cell and at the same time the nu- 

 cleus, spindle and surrounding cytoplasm 

 are carried bodily toward the surface of the 

 egg. Coincidently with this migration of the 



nuclear constituents there is a segregation 

 of the cytoplasm at one pole (the animal) 

 and of yolk at the other (the vegetal). This 

 separation of yolk and cytoplasm goes on 

 during the second maturation division and 

 throughout all the stages of fertilization. 

 The movements of the germinal vesicle 

 and of the maturation spindles, the separa- 

 tion of yolk and cytoplasm and also the ap- 

 proach of the pronuclei during fertilization 

 seem to be due to protoplasmic currents. 



In the cleavage of the egg the evidence 

 for such currents is much more abundant 

 and complete. Centrosomes and Zwischen- 

 korpern are preserved throughout the rest- 

 ing period following division, and by means 

 of the relative positions of these bodies at 

 different stages, as well as the relative po- 

 sitions of the nuclei, yolk and cytoplasm, 

 the direction and extent of these movements 

 can be accurately determined. During the 

 anaphase of the first cleavage the spindle 

 lies at right angles to the egg axis, and the 

 centrosomes, chromatic plates and Zioigchen- 

 korper are in a straight line. In later stages 

 the Zwischenkorper is carried down to the 

 center of the egg, the centrosomes are car- 

 ried up to the surface and move toward 

 each other until they come to lie on each 

 side of the first cleavage plane and imme- 

 diately under the polar bodies ; the nuclei 

 are also moved upward and toward each 

 other until they are almost in contact on 

 opposite sides of the first cleavage wall, and 

 the cytoplasm moves down into the center 

 of the egg, the yolk at the same time mov- 

 ing up at the periphery. Such movements 

 could be caused only by vortical currents 

 in the daughter cells moving up at the sur- 

 face and down through the center pf the 

 egg ; the cell wall forms where these oppo- 

 site currents meet. 



Similar vortical currents occur in every 

 cleavage up to a late stage, and they ofier 

 most important evidence not only as to the 

 mechanics of cleavage, but also as to the me- 



