220 BIOLOGY: GENERAL AND MEDICAL 



reproductive organs which preside over the vegetative 

 functions. 



This chief axis may be recognized, in large mero- 

 blastic eggs, even before development begins, and in 

 many cases is distinct as soon as it begins; thus, in the 

 hen's egg, the germinal vesicle represents the animal 

 pole, the great opposed mass of the yolk the opposite or 

 vegetative pole. 



All of the metazoa early show a monaxial, hetero- 

 polar condition about which the developmental process 

 centres. In eggs with yolks the animal cells are lighter 

 than the vegetative cells, so that the animal pole always 

 turns up, no matter in what position the egg is placed. 

 In eggs without yolks and with equal or fairly equal 

 cleavage the animal cells distribute over the surface and 

 the vegetative cells arise within, so that the vegetative 

 pole is central. 



The process of cleavage takes place through karyo- 

 kinesis, the plane of division being perpendicular to the 

 long axis of the spindle. Two cells are thus produced, 

 each of which appear to possess an equal amount of 

 reproductive energy and an equality of all the factors 

 concerned in development, for it has been found by 

 experiment that if these halves can be separated and 

 the developmental process continued, as is possible 

 with some of the lower animals, each is able to pro- 

 gress without apparent serious disturbance to complete 

 development. 



Further cleavage results through further karyokinesis, 

 the poles of the nuclear spindle always being directed 

 toward the greatest protoplasmic masses, so that there 

 result four, eight, sixteen, thirty-two, sixty-four, 128 

 cells, or blastomeres, and so on. This process of cleavage, 

 though taking place after karyokinetic changes, differs 

 from ordinary cell division in that it progresses so rapidly 

 that no time is allowed for growth and the cells become 

 smaller and smaller as they divide. 



In equal cleavage the cells of the two poles are of 

 uniform size; in unequal cleavage their number is the 



