384 THEORIES OF DEVELOPMENT 



membrane generally disintegrates and the chromatin thread is 

 broken up into U-shaped fragments (chromosomes). These 

 U-shaped chromosomes are arranged as the equatorial plate 

 half-way between the centrosomes and across the axis of the 

 spindle. Either before or after the formation of the equatorial 

 plate the chromosomes split longitudinally, so that each part 

 contains an equal amount of the chromatin. The chromosomes 

 now move along the spindle (or its fibers contract), so that one- 

 half of each original chromosome is drawn to one centrosome 

 and the other half to the other centrosome. These chromo- 

 somes, with their apexes pointing toward the center, are now 

 arranged about their respective centrosomes. The protoplasm 

 of the general cell becomes constricted in the center, each group 

 of chromatin loops rearranges itself into a nucleus like that of 

 the mother cell, and an investing membrane becomes ap- 

 parent. A furrow appears on the surface of the protoplasm and 

 gradually deepens until the protoplasm is divided into two equal 

 segments, each containing its own nucleus and centrosome and 

 being a complete daughter-cell. 



Maturation. — Metazoans usually reproduce by means of 

 fertilized eggs or ova. Egg cells vary in size from less than utt 

 to about T^¥ inch in diameter. By this is meant the minute 

 germ cell without the enormous amount of nutritive material 

 which usually accompanies it. 



Before the process of fertilization takes place (about the 

 time of the entrance of the spermatozoon, which, howe^"er, 

 takes no part in the process) the ovum undergoes a process of 

 maturation, in which one-half of the number of chromosomes is 

 thrown off. A spindle-shaped structure is formed from minute 

 fibers in the cytoplasm. The centrosomes, one at each pole of 

 the spindle, seem to control its formation and acti^•ities. This 

 spindle draws from the egg nucleus a definite number of chro- 

 mosomes which are arranged across the center of the spindle, 

 which now moves endwise toward the surface of the egg. A 

 small protrusion is made, the spindle divides across the center, 

 and the first polar cell, consisting of a nucleus with chromo- 

 somes and a small amount of cytoplasm, is formed and thrown 

 off. The remaining portion of the spindle disappears. The 

 other centrosome divides into two and forms another sjiindle, 



A' 



