66 



FUNDAMENTALS OF CYTOLOGY 



remains in the intervening fluid. Strictly .speaking, cytokinesis, the 

 division of the cytosome, has occurred as soon as the halves of the original 

 protoplast are capable of rounding up as two inde]:)endent protoplasts, 

 for to do this each of them must have completed its plasma membrane 

 on the side next to the cell plate. The cell plate, with certain modifica- 

 tions, remains as the intercellular substance, or middle lamella, upon 



1' urination of cull ijlato in T radvscantia stamen 

 hair. (After W. A. Becker.) 



Fig. 46. — Phraginoplast 

 with cell plate in iris endo- 

 sperm. (After V. J lingers.) 



which the cellulose wall layers are deposited, thus completing the parti- 

 tion which separates the new protoplasts in plant tissues of this kind. 

 The development and nature of the cell wall will be described in the 

 next chapter (page 75). 



SOMATIC CELL DIVISION IN ANIMALS 



Somatic division in animals differs in many cases from that in most 

 plants in two conspicuous features: (1) the achromatic figure is often 

 much more elaborate, having a pair of asters at the spindle poles and 

 commonly a centrosome at the focus of each aster; (2) cytokinesis is 

 accomplished by a furrow which progresses inward from the periphery 

 of the cell, rather than by a cell plate originating in the middle and 

 extending to the periphery. 



Mitosis. — Chromosome behavior during mitosis in animals is essen- 

 tially like that in plants. The same series of principal phases is passed 

 through, and the main significant result is the same: the division of the 

 nucleus is equational, the original nucleus and the two daughter nuclei 

 all being alike in chromosomal composition and functional capacity. 

 In recent years chromonemata have been studied less in animals than 

 in plants. Animal and plant mitoses may differ in minor ways, but it 

 seems likely that the two kingdoms will not be found to disagree widely 

 in any very fundamental feature of chromosome behavior. 



