vi] POLAR DIVISIONS 79 



Those which go to the outer pole at the egg-margin form a 

 nucleus which is pushed out of the egg, possibly by the 

 elongation of the spindle, together with a very small amount 

 of cytoplasm; this constitutes the "first polar body." The 

 other group goes to the inner pole of the spindle, and 

 usually also forms a vesicular nucleus for a short time. In 

 most animals the second division follows rapidly, though 

 in some a considerable interval may elapse; a new spindle 

 is formed, the egg-nucleus again is resolved into chromo- 

 somes which split and separate as in the homotype division 

 of the secondary spermatocyte. Again the outer group of 

 chromosomes travels to the edge of the egg and is extruded 

 as a second nucleus enclosed in a small mass of cytoplasm, 

 constituting the second polar body. Sometimes, but not 

 in all animals, the first polar nucleus divides concurrently, 

 so that the whole process gives rise in all to three minute 

 polar cells and one large nucleated egg-cell. The nucleus of 

 the egg-cell, sometimes now called the " female pronucleus," 

 then sinks into the egg and is ready for conjugation with 

 the sperm-nucleus, which in many animals is already present 

 in the egg at this stage. 



The maturation phenomena in the egg are thus closely 

 comparable with those of the spermatozoon. The early 

 stages are very similar ; then follows a period of growth and 

 usually of nuclear change not represented in spermato- 

 genesis, but when this is completed two successive nuclear 

 divisions occur, giving rise in all to four cells as in spermato- 

 genesis (cf. diagrams in Text-fig. 7, p. 61). There is, 

 however, the conspicuous difference, due to the great size 

 and elaboration of the egg, that instead of these cells 

 all being similar, one is a large, functional germ-cell, while 

 the other three are minute bodies which in most cases are 

 thrown out of the egg and lost. The differences, how- 



