FOOD AS A FACTOR IN THE DETERMINATION OF SEX. 45 



from the females with absolute certainty without making a his- 

 tological examination of the gonads. The method used to dis- 

 tinguish the sexes in very young toads was as follows : the toad 

 was placed in a flat, shallow, dish containing a layer of paraffine 

 which makes an excellent surface for cutting, and the body cavity 

 was then opened under a dissecting lens ; the kidneys and the 

 genital organs attached to them were removed by means of small, 

 sharp knives, and subsequently, under a much stronger lens, the 

 gonads were examined in toto. Figs. I and 2 show the differ- 

 ences between the gonads of the two sexes in young toads that 

 have recently completed metamorphosis. The testes (Fig. 1) are 

 at this time about 2 mm. in length, they are relatively narrow, 

 cylindrical bodies with a smooth outline ; the ovaries (Fig. 2), 

 on the contrary, are usually broader than the testes and they 

 have an irregular, jagged outline. Bidder's organ (Figs. 1 and 

 2, B) is very prominent in all individuals at this time ; but as it 

 is practically the same size in both sexes, it is of no aid in dis- 

 tinguishing males from females. 



In order to ascertain whether the external appearance of the 

 genital organs (as shown in Figs. I and 2) is a positive indication 

 of the sex of the individual, 50 young toads were selected of which 

 25 had gonads approximately like those shown in Fig. 1, and 25 

 had gonads similar to those in Fig. 2. The gonads were stained in 

 toto with hematoxylin and sectioned. The histological examina- 

 tion proved conclusively that the external appearance of the 

 gonads can be relied on to indicate the difference in sex, as 

 the sections showed unquestionably that there were 25 males in 

 the one lot and 25 females in the other. At the time of meta- 

 morphosis the genital organs are not equally well developed in 

 all individual, however, and occasionally it is impossible to dis- 

 tinguish the sex of a toad without making use of sections. 



All of the tadpoles that died during the course of the experi- 

 ments were fixed in corrosive-acetic (5 per cent, acetic acid) if 

 the hind legs were well developed and the sex ascertained, when 

 possible, by means of sections. A histological examination of the 

 gonads enables one to ascertain the sex of a tadpole some time 

 before the front legs have appeared ; for, although the germ-cells 

 may appear similar at this time, the ovary has a central cavity 



