4 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



egg cell, a change takes place in the latter which prevents the entrance 



of others. 



SEXUAL MATURITY. 



Animals reproduce only during the part of their lifetime following 

 sexual maturity and preceding senescence. During this period most 

 of them reproduce only in a certain season of the year, the breeding 

 season, and within this season a given female will breed during only 



Fig. 1. — A group of dividing reproductive cells from a male grasshopper. The curious spindle-shaped 

 figure, which is characteristic of dividing cells, is clearly brought out. In the formation of the repro- 

 ductive cells, the chromosomes, which appear solid black in the illustration, come together in pairs 

 around the equator of the spindle, separate and move to opposite poles, forming two nuclei as a pre- 

 liminary to division ofthecellas a whole. Each of the resulting cells has only half the original number 

 of chromosomes. In ordinary cell divisions, each chromosome splits, the halves moving to opposite 

 poles, each of which thus obtains the same number of chromosomes as the original cell. Only a few of 

 the chromosomes are in focus in the picture, which was taken through a high-power microscope. (Cour- 

 tesy of Dr. E. E. Carothers.) 



a very brief period, the period of heat or oestrus. In many cases the 

 oestrous period recurs at definite intervals during the breeding season. 

 The age of maturity, while in a general way characteristic for a 

 given kind of animal, depends on a great number of factors in indi- 

 vidual cases. A warm climate, liberal feeding, and good care in 

 general are conducive to early maturity. There are also hereditary 

 differences. Early sexual maturity is doubtless correlated with the 



