108 CYTOKINESIS. 



all future development. Through this pole the first two cleavage furrows always 

 pass ; around it three quartettes of ectomeres are formed, each of which has a definite 

 developmental history and gives rise to definite parts of the larva or adult ; certain 

 of these blastomeres are visibly different from each other in size, position, shape and 

 quality, and although these differences arise gradually in the course of development, 

 the polarity of the unsegmented egg exercises a determining influence ujson all of 

 them. This jDolar differentiation of the egg is therefore of the greatest prospective 

 significance. 



The eccentricity of the germinal vesicle, which is the earliest evidence of this 

 polarity in the free egg, is itself, most probably, the result of polarity^ already existent 

 in the cell from which the egg is formed. This polaritj' must be regarded as the 

 factor which directs the general cell movements which bring about the segregation 

 of yolk and cytoplasm ; while the immediate cause of these movements is very 

 probfibly the escape of achromatic substances from the nucleus. 



In the fertilization of the egg the same sorting of the egg contents continues as 

 in the maturation, w4th the result that at the beginning of the first cleavage almost 

 all of the yolk is collected at the vegetal pole while the greater part of the cyto- 

 plasm lies close around the animal pole. 



[b] Blastomeres. Every blastomere manifests the same type of polarity as 

 the unsegmented egg itself At every cleavage this polarity of the blastomeres is 

 lost or modified, only to be reestablished again in each telophase. In every divi- 

 sion of blastomeres containing yolk the mitotic figure surrounded by cytoplasm 

 moves down into the yolk area while the yolk moves up at the periphery toward 

 the animal pole. In the teloj^hase and resting pei'iod, however, the centrosomes, 

 nuclei and cytoplasm again take a superficial position near the animal pole, while 

 the yolk again moves toward the vegetal pole. This polar movement concerns not 

 only cytoplasm and yolk but also different kinds of protoplasm. Thus the sphere 

 substance always takes a definite polar position in each blastomere and the localiza- 

 tion of certain characteristic kinds of cytoplasm (hyaline or granular) is also referable 

 to polarity. 



In each blastomere the cell axis (Heidenhain), by which is meant the line 

 passing through the center of the nucleus and centrosome, shifts during the telo- 

 phase until the centrosome and sphere are carried to a free surface of the cell and 

 as close as possible to the animal pole. Tlie result is that in all the early cleavages 

 the cell axis tends to become parallel with the original egg axis, though this is 

 prevented in many instances by other cells Avhich lie nearer the animal ^Dole. 



{c) Nucleus, Centrosome and Sphere. — During the rotation of the cell con- 

 tents in the telophase the spindle axis rotates as a whole so that the nucleus at all 

 stages in this rotation presents approximately the same side (its central pole) toward 

 the centrosome and sphere, and its opposite side (distal pole) toward the mid-body. 

 Throughout this rotation the centrosome and sphere also present the same side 

 toward the nucleus. The polarity therefore of the nucleus, centrosome and sphere, 

 as well as that of the cell body, is reestablished after every division. 



