984 THE PHYSIOLOGY OF REPRODUCTION. 



division is followed by a separation of the cytoplasm into two 

 parts. Thus, two daughter-cells are formed, each containing the 

 same number of chromosomes as the parent cell, but only half the 

 amount of chromatin material. In ordinary cell division the 

 chromosomes again form a resting reticulum and a nuclear 

 membrane, and the chromatin substance increases in quantity. 

 The primitive egg-cells, or oogonia, divide according to this 

 schema, but before an ovum is ready for fertilization it undergoes 

 a process of maturation which consists in two successive cell 

 divisions differing somewhat from the type just described. Pre- 

 ceding the first of these divisions the various chromosomes come 

 together in pairs and fuse more or less completely, the process of 

 conjugation being designated as the period of synapsis. The 

 evidence indicates that in this pairing of the chromosomes one of 

 each pair was originally maternal in its origin, the other was 

 paternal. The result of the conjugation is to reduce the number 

 of chromosomes typical for the species by one-half. In man, for 

 example, the typical number of chromosomes for the tissue cells 

 is 48; by the synapsis they are reduced to 24. Following the 

 synapsis the ovum undergoes the first of the maturation divisions, 

 sometimes known as the reduction division. The two ova pro- 

 duced are very unequal in size; the smaller one is known as the 

 first polar body, and it subsequently degenerates. The larger 

 one is the ovum, but it now contains only half the typical number 

 of chromosomes, since in the division the paired chromosomes 

 separate, one of each pair going to the polar body. The separation 

 of the paired chromosomes is such that the ovum gets some that 

 were of maternal and some that were of paternal origin. After 

 the formation of the first polar body the ovum again divides, this 

 time in a typical way, with an equal splitting of the chromosomes, 

 except that again there is a great disparity in size, the smaller 

 cell being known as the second polar body.* Since the first 

 polar body after its separation may again divide into two cells, 

 the process of maturation results in the formation of four cells, 

 three of which are polar bodies and may be regalrded as abortive 

 ova. The fourth, the matured ovum, retains practically all of 

 the original cytoplasm, but has lost a part of its chromatin mate- 

 rial and, according to Boveri, also its centrosome. The production 

 of these four cells may be represented, therefore, by a schema of 

 the kind shown in Fig. 304. 



From a biological standpoint the reduction of chromosomes 

 throws much light upon the significance of fertilization by the male 

 cell. The spermatozoon before it is ripe undergoes a process of 



* Wilson, "The Cell in Inheritance and Development," also Conklin, 

 "Heredity and Environment," 1914. 



