44 ZOOLOGY 



and the oogonia produce eggs, there are two important happen- 

 ings: (a) the union of the simple chromosomes of a nucleus, 

 originally derived from the two parents, to form the bivalent 

 chromosomes; and (b) the separation of these at the next division 

 in such a way as to reduce the simple chromosomes to one-half 

 the number usually found in the cells of the species (Figs. 12, 13). 



It will be recalled that the chromosomes found in any primor- 

 dial germ or body cell are descendants of chromosomes that came 

 in equal numbers from the egg and sperm nuclei when they first 

 united. We have seen that every cell gets its part of every one of 

 these chromosomes. It is believed, when these chromosomes 

 unite in pairs in the act of maturing, that they do not unite in a 

 chance manner, but that those pairing are corresponding chromo- 

 somes coming originally from the mother and father. In other 

 words this union is the final scene in the mixing of male and female 

 qualities which began when the nuclei united in fertilization. 

 The protoplast of ovum and sperm unite at fertilization; but 

 the final union of the chromosomes of the ovum and sperm is 

 delayed until near the time of the next union of sperm and ova. 

 It is possible that these bivalent chromosomes may be exchang- 

 ing material before their final separation. 



The reduction of the chromosomes to one-half their typical 

 number is clearly a device which maintains the standard number 

 when the ovum and sperm unite. If they were not reduced the 

 amount of chromatin, and the accompanying hereditary effects, 

 would be doubled at every fertilization. 



61. Summary. 



1. All the higher animals begin life as a single cell and reach 

 their adult condition by a continuous series of divisions. By 

 the growth and specialization of the cells . arising from these 

 divisions the great complexity of the adult body is produced. 



2. This initial cell the fertilized ovum represents the 

 fusion of two independent and unlike cells: the ovum (female) 

 and the spermatozoon (male). 



3. Before the union (fertilization) occurs, the ovum reduces 

 its nuclear material, by two successive divisions, to one-fourth 

 its original amount and the chromosomes to one-half their 



