DEVELOPMENT OF A NON-AQUATIC TADPOLE. 259 



the sacral diapophyses. For example, in Boulenger's sketch of 

 the skeleton of Alytcs obsteiricansf the ends of the ilia reach 

 the level of the transverse processes of the vertebra in front of 

 the sacrum. This stretching forward of the ilium is interesting 

 in connection with the view that the sacrum has advanced 

 forwards during the phylogeny of the Anura; and in existing forms 

 the sacrum may consist of one, or more, of vertebrae X — VI. 



In the adult there is a firmisternous pectoral girdle, clavicles 

 absent and coracoids widely expanded ventrally. In the tadpole 

 the girdle arises in a firmisternous condition and the coracoids are 

 broadened ventrally. In the large tadpole there is a well-marked 

 pre-coracoid cartilage, but in the transforming frog it is already 

 becoming greatly attenuated. 



Dimorphism of the Eggs and Embryos. 



A careful examination of all the eggs and embryos disclosed 

 the fact that they could be grouped into two series of about equal 

 numbers. In the one series the diameters of the eggs ranged 

 from about 1"7 to 2-2 mm. and in the other from about 2-4 to 

 2"7 mm., the average diameters being about 2'0 and 2"6 mm. 

 respectively, and the cubic content being in the ratio of 1:2. 

 These eggs developed into small and large tadpoles in which 

 there was naturally a very marked difference in the size of the 

 yolk-mass. In the case of the developing tadpole of the small-sized 

 series the tail very early projects straight out from the yolk-mass 

 in line with the long axis of the embryo, while in the large-sized 

 series the tail grows round the yolk-mass and does not straighten 

 out until much later in development (PI. II, figs. 3, 4). 

 Taking the development of the hind limbs as a criterion 

 of age, and comparing the tadpoles of similar ages in 

 the two series, it is found that there is a marked 

 difference in the tails. In the smaller tadpoles the tail 

 is relatively deeper dorso-ventrally and the swimming membranes 

 wider than in the larger tadpoles. It would appear that a more 

 normally developed swimming membrane is associated with a less 

 excessive quantity of yolk. Since large and small eggs occurred 

 in the same clutch, and the two sizes were approximately equal 

 in number in the whole series, it would appear very probable that 

 the differences are associated with sex, and that the small eggs 

 develop into males and the large eggs into females. On first 

 observing the difference in the tadpoles it was not realised that 

 marked variation in the size of the eggs occurred in the same 

 clutch, and I consulted Mr. Hewitt as to the possibility of the 

 eggs being mixed and belonging to two allied species. Mr. Hewitt 

 re-examined his series of specimens and came to the conclusion 

 that only one species was present. That the dimorphism of the 

 eggs and tadpoles is associated with sex is rendered more probable 

 by the fact that there is an unusual amount of difference between 

 the adult male and female. The males tend to be smaller, they 



t Bonlenger, G. A. — I.e., p. 167. 



