244 WILBUR WILLIS SWINGLE 
tene, and diplotene, are said to occur normally extremely early in 
most vertebrates, in some mammals before birth. In male indi- 
viduals, according to the usual accounts in the literature, the 
same stages of the maturation cycle do not usually take place 
until shortly before the attainment of sexual maturity. 
It has long been known that the germ cells of female frogs 
undergo the earlier maturation changes while the animal is still 
a tadpole. The growth period of the oocyte in this group of 
amphibia is assumed to last a very long time, though further 
work may show this not to be altogether true. According to 
several investigators of European frogs, the eggs are not ready 
for fertilization until the fourth or fifth season after metamor- 
phosis, when the first polar body is extruded shortly before fer- 
tilization. In this connection Gatenby ('14) says of Rana 
temporaria : 
Though one camiot be certain, I believe that an oocyte takes two 
years at least, and more probably three to become mature. It is 
evident, therefore, that the young oocytes formed in April or May in 
the adult will not be used for spawning next March, but certainly for a 
spawning several years ahead. The first ova derived from primordial 
germ cells would not be spawned till three j^ears after the hatching of 
the tadpole, since the frogs around Oxford seem to become mature in 
three years. 
However this may be, the germ cells of male Rana catesbeiana 
larvae enter maturation simultaneously with those of female tad- 
poles — long before metamorphosis, or before the gonads have 
even differentiated sufficiently to resemble a testis. Until the 
onset of maturation the gonads of the two sexes are morpho- 
logically identical. Following the precocious maturation cycle, 
it becomes easy to differentiate the sexes, as the female germ 
cells soon enter the growth period and become oocytes. The 
gross appearance of the glands of the two sexes changes at this 
time. The male germ cells pass through all stages of maturation, 
leptotene, amphitene (synapsis), pachytene, diplotene, tetrad 
formation, up to the first maturation division in a perfectl}^ 
normal manner. During the anaphase of the heterotypic mitosis, 
or at the earlier period of spindle formation, the spermatocytes 
