REPRODUCTIVE ORGANS IN ANIMALS 37 



out, the nucleus becoming shifted to one end and forming the head 

 of the spermatozoon, which is then set free. 



The maturation of the ovum differs from the corresponding 

 process in the spermatozoon in the unequal division of the cell. 

 In the first division, which is the " reduction division " (the 

 chromosomes not splitting), the first polar body is extruded. 

 This consists of half the nuclear material of the ovum, invested 

 by a thin layer of external protoplasm or cytoplasm. The second 

 division results in the formation of the mature ovum and the 

 second polar body, this consisting, like the first, almost exclusively 

 of nuclear material. In this division, as with the second division 

 in spermatogenesis, the chromosomes undergo the usual splitting. 

 The maturation process is commenced when the ovum is still 

 inside the ovarian follicle, before ovulation (or the discharge of 

 the ovum), and is continued and completed while the ovum is 

 in process of passing down the Fallopian tube. Sometimes the 

 first polar body divides into two equal halves, so that there are 

 altogether three polar bodies, all of which die and are absorbed, 

 leaving the mature ovum, with the number of chromosomes 

 reduced to one-half, alone remaining. 



It is stated that prior to the formation of the first polar body 

 the chromosomes of the ovum unite together in pairs, and that 

 during this process, which is called "synapsis," there is an exchange 

 of granules between the chromosomes. A sirnilar process is said 

 to occur in connection with spermatogenesis. The hereditary 

 characteristics are believed by many to be located in these 

 granules, so that it is a matter of chance whether a particular 

 character remains located in a chromosome, or whether it passes 

 into another chromosome which may be ejected. In this way 

 an attempt has been made to explain the manner of distribution 

 of the hereditary characters among the different members of a 

 family. 



The total number of chromosomes characteristic of the cells 

 of a particular species is restored once more by the conjugation 

 of the ovum with a spermatozoon This usually occurs in the 

 Fallopian tube, the two nuclei becoming fused while the tail of 

 the spermatozoon after entry into the egg becomes disintegrated 

 and mingles with the latter 's cytoplasm. 



