36 INTRODUCTION. 



CELL-CLEAVAGE. 



The process of cell-formation by means of a progressive cleavage is 

 best known at present in certain Phycomycetes and Myxomycetes. As 

 a convenfent and suitable illustration of this method the process of 

 cleavage leading to spore formation in the sporangium of Synchitrium, 

 parasitic upon the hog peanut, and of Sporodinia is selected. For 

 our knowledge of cleavage we are again indebted to the researches of 

 Harper ('99). 



The so-called initial cell of the sporangium of Synchitrium, when 

 almost fully developed, is large enough to be visible to the unaided 

 eye, and contains a relatively large nucleus (Fig. 14, A). This nucleus 

 divides several times until a large number of nuclei are present, which 

 lie irregularly distributed in the cytoplasm. 



Cleavage of the cytoplasm now begins. It does not take place by 

 repeated bipartitions, nor by the simultaneous precipitation of a cell- 

 wall about each nucleus. As mentioned in a preceding paragraph, it 

 resembles in a large measure the process in certain animals, as for 

 example, the dividing protoplasm of the germinal disk of the chick, 

 or perhaps more nearly that in certain insect eggs in which a series of 

 nuclear divisions precedes cytoplasmic segmentation. 1 



The cleavage begins by the formation of furrows on the surface, 

 which grow deeper and deeper in a direction more or less radial. It 

 is progressive and divides the cell into successively smaller portions 

 (Fig. 14, D). The process is described in detail by Harper as follows : 



These grooves are in reality so narrow as to appear as plates, which grow 

 wider by additions along their inner margins till they intersect, and thus divide 

 the protoplasm into irregular blocks or sometimes pyramids with their bases in 

 the surface of the initial cell (Fig. 14, D, E). Only at the very periphery the 

 separation of the cut surfaces of the protoplasm to form a shallow notch, as 

 it appears in section, reveals the true nature of the process as a pushing in of 

 the free surface to form a deep though extremely narrow constriction. 



In many cases there is at first no separation of the newly formed surfaces ; 

 they remain closely appressed, up to the periphery of the cell. The groove 

 appears in section, merely as a single line which the Zeiss appochromatic lens 

 1.40 ap. fails to resolve into two closely appressed surfaces (Fig. 14, B). The 

 position of the line is further emphasized by the arrangement of the vacuoles, 

 which are pushed aside and form in section two more or less regular rows in 

 the plane of the newly formed surfaces on each side of the furrow. Such a line 

 might be taken for a cell-plate which subsequently splits to form the boundaries 

 of the protoplasmic segments or which is metamorphosed into the cellulose 

 walls of the spores. That this line, however, in reality represents from the start 

 two closely appressed surfaces is abundantly shown in many cases. 



1 Hcrtwig: Die Zelle und die Gewebe, p. 187. 



