470 



AN AMERICA* TEXT- BOOK OF PHYSIOLOGY. 



original idea of Van Beneden, 1 that the astral raws are contractile and 

 mechanically pull apart the half-chromosomes, is supported by considerable 

 lait unconvincing evidence. The idea appears to be growing that by reason 

 of chemical changes taking place in the centrosomes the halt-chromosomes 

 are attracted t<> the two poles of the spindle. 2 Strasburger 3 suggests that 

 this attractive influence is chemotaxis. In the process of division each 

 nuclear half obtains half of the original male and half of the original 

 female chromatin, and hence contains inherited potentialities of both parents. 

 After division each half gradually assumes the structure of a typical resting 

 nucleus with its accompanying aster (Fig. 226). 



The third act of segmentation consists of a simple division of the cytoplasm 

 into two ecpial parts, the separation taking place along the plane of nuclear 



/ 



/ 



I Jrar 





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\ 



/ 



\ 



i 



i 

 i 



/ 



Fir,. 226.— Stages in the segmentation of the egg (continued from Fig. 226). 



d. The vesicular chromatic masses have become converted into two typical resting nuclei, each with 

 n chromatic network. The single aster, formerly connected with each nuclear mass, bas become divided 

 into two, which have taken positions at opposite poles of the nuclei. The division of the cytoplasm is 



complete, and the two resulting cells, or blastomeres, are resting, preparatory to a second division in a 

 plane at right angles to that of the first. 



division (Fig. 226, i>). Each part contains one of the new nuclei, and the 

 result of the firsl division is the existence of two cells, two blastomeres, in 

 place of the one fertilized ovum. The beginning of differentiation is often 

 shown even as early as this, for one blastomere is often somewhat larger and 

 less granular than the other. 



Each blastomere proceeds now to divide by a similar mitotic process into 

 two. the resull being four in all, and by subsequent divisions, eight, sixteen, 

 and more, the division- not proceeding, however, with mathematical rcgu- 

 laritv. By such repeated mitotic processes the original fertilized ovum 

 becomes a mas- of small and approximately similar cells, the morula, from 

 which by continued increase in the number of the cells, morphological differ- 

 entiation, and physiological division of labor, the embryo with all its functions 

 is destined to be built up. 



'Van Beneden: Arclt'n-rs ck Iliolat/k, iss.'j, iv. 



1 - v . Biitechli: Verh. NalurhwL med. Ver. Heidelberg, 1891 ; and E. B. "Wilson, op. cit. 



'Strasburger: Analomi&cher Anzeiger, 1893, viii. 



