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 A 
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 
.—A group of dividing reproductive cells from a male grasshopper. The curious spindle-shaped 
li^which is characteristic of dividing cells, is clearly brought out. In the formation of the repro- 
of chills, the chromosomes, which appear solid black in the illustration, come together in pairs 
poles, eacfi^ 01, of ttLe spindle, separate and niofe to opposite poles, forming two nuclei as a pre- 
the ehromosoJP of thecel l as a '""hole. Each of the resulting cells has only half the original number 
fcesy ofDr E F P rc ^ nai T cell divisions, each chromosome splits, the halves moving to opposite 
^ obtains the same number of chromosomes as the original cell. Only a few of 
p TrMm- T~ • n "in the picture, which was taken through a high-power microscope. (,Cour- 
a rer ^ Dp ief period b 
oestrous period recurs At 
The sap j ^ , . ' ">d of heat or oestrus. In manv cases the 
7-~ j " - m -^ intervals during the breeding season. 
ol general way characteristic for a 
e, libtgat number of factors in indi- 
f maturityfeeding, and good care in 
s-ivpn l^Cj n — w ™ny, wnn, 
S^en Kmd of animal d 
^idual cases A ' ae pends oj^ general wav characteristic for a 
vidual cases, 
gen 
differences. 
7 — ^vxiut, oj^ general way cnaractenstic tor a 
ran climate, libsgat number of factors in indi- 
sexual maturity is doubi e , re are als0 hereditary 
correlated with the 
