236 WILBUR WILLIS SWINGLE 
INTRODUCTION 
Several years ago, while engaged in experimental work involv- 
ing the germ glands and germ cells of anurans (Swingle, '17, also 
'17-' 18), the writer was somewhat hampered by lack of definite 
criteria for differentiating the sexes in young larvae. In so far 
as the cytological conditions presented by the germ cells were 
concerned, it was impossible at that time, to distinguish clearly 
male from female tadpoles. The literature concerning sex in 
larval Anura was found to be voluminous and contained a great 
variety of opinions, many of which were mutually exclusive, 
others evidently based upon scanty evidence of somewhat dubi- 
ous value, and none in any sense adequate to account for the 
conditions presented by my material. In the summer of 1917, 
therefore, an attempt was made to clear up the puzzhng ques- 
tion of sex differentiation, but the effort proved abortive owing 
to lack of sufficient material. Certain cell stages occurred in my 
larval material which had been a source of mystification to the 
writer and to many others as well who had examined the mate- 
rial; these stages had apparently never been observed or at any 
rate reported by previous workers on anurans. Fortunately, 
an opportunity soon presented itself of working with Prof. E. G. 
Conklin, of Princeton University, who made a suggestion that 
further investigation has since shown to be correct, i.e., that I 
was dealing with a precocious maturation cycle in anuran lar- 
vae. Professor Conklin's suggestion throws an entirely new 
light upon the question of sex differentiation and development 
in the Anura, and brings the sexual conditions of these forms 
more nearly into line with those described for other vertebrates. 
It is a pleasure to acknowledge my indebtedness to Professor 
Conklin for this illuminating suggestion, and for many others as 
well, which have made this work possible, for the time he has 
spent looking over material, and for the keen interest displayed 
in the progress of the work. 
To Prof. N. P. Sherwood and Dr. Cora Downs, of the Depart- 
ment of Bacteriology of the University of Kansas, I am greatly 
indebted for aid in collecting 2000 tadpole specimens from the 
outlying districts of Douglas County, Kansas, during the summer 
