~4 



THE MAMMALIAN EGG 



radial orientation (Fig. 63) ; one chromosome group thus comes to 



lie nearer the centre of the egg 



■Kfcfr* 



Fig. 63 

 Movements shown by the telophase 

 second-meiotic spindle of a recently 

 penetrated rat egg while under observa- 

 tion in vitro. X 1,400. 



while the other remains close to the 

 surface. (Spindle rotation occurs in 

 rodents and some other animals, but 

 may not do so in all mammals. O. 

 Van der Stricht (1909), R. Van der 

 Stricht (191 1), Pearson and Enders 

 (1943) and J. L. Hancock (personal 

 communication, i960) maintain 

 that the spindle is always radially 

 orientated in the bat, cat, fox and 

 pig, respectively.) Concurrently, 

 the surface indentation deepens and 

 extends around the external pole of 

 the spindle so as to cut off the small 

 body of cytoplasm that contains 

 the more superficial chromosome 

 group. The cytoplasm composing 

 the polar body is generally charac- 

 terized by the presence of few 

 granular elements. For a while 

 after its formation the polar body 

 remains connected to the vitellus by 

 the spindle which can be shown by 

 manipulation to have appreciable 

 tensile strength (Odor and Blandau, 

 1951a). When the spindle is finally 

 transected, separation occurs just 

 medially to the intermediary body 

 (Blandau, 1945; Ward, 1948; Odor, 

 1955; Austin, I956d); the rna shed 

 by the chromosomes is thus jetti- 

 soned in the polar body. 



In many non-mammalian ani- 

 mals, the first polar body divides 

 into two so that three polar bodies 

 are eventually formed; this is rare 

 in mammalian eggs, but has been 

 reported (Sobotta, 1895; Rubasch- 

 kin, 1905; Krassovskaja, 1934; 



