STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 73 



Several theories have been advanced to explain the mechanism of 

 cell division and these have been systematically reviewed by Swann 

 and Mitchison (1958) ; a detailed account is given also by Ris (1955). 

 Briefly, opinions are as follows. The initial elongation of the egg 

 could be attributable to extension of the spindle and the mechanical 

 effects exerted by the asters. These effects seem more likely to be 

 caused by traction by astral fibres attached to the surface of the cell, 

 pulling in the surface in the region between the spindle poles, rather 

 than by pressure against the surface external to the spindle poles. 

 A contributory factor leading to the dipping in of the cleavage 

 furrow may possibly be an alteration of surface properties in the 

 central region caused by some agent emanating from the breakdown 

 of the nucleus. Since cleavage necessarily involves considerable 

 increase in the area of the cell cortex, it is suggested that the motive 

 force for cell division may well be a passive extension of the cortex 

 brought about by addition of material in the regions external to the 

 spindle poles, the material possibly originating from the polar 

 groups of chromosomes. Associated with such a process, there is 

 almost certainly an active growth of the cortex in the depths of the 

 cleavage furrow, particularly during its terminal movements. 



Polar-body Emission 



Early views on the function of polar bodies included the sugges- 

 tions that they served as cushions to protect the vitellus (Rabl, 1876), 

 that they were a means of disposing of unwanted material (Semper, 

 1875 — 'a form of defaecation' ; Fol, 1875), and that they were 

 rudimentary cells having an atavistic significance (Giard, 1877) 

 (references cited by Blanchard, 1878). They were widely thought 

 to determine the direction of the cleavage furrow, which in many 

 non-mammalian eggs clearly begins at the animal pole near which 

 the polar bodies remain. 



Emission of the polar body takes place after the meiotic division 

 has reached telophase, and follows much the same course with both 

 first and second polar bodies (Fig. 14). Initially, the telophase spindle 

 lies just below the surface of the egg and in a plane parallel to the 

 tangent. The first visible sign of polar-body formation is an indenta- 

 tion of the egg surface at a point immediately peripheral to the 

 equator of the spindle, which is marked by the presence of a very 

 distinct intermediary body. The spindle then moves inwards and 

 rotates about one pole until its long axis assumes approximately a 



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