THE LOWER VERTEBKATA. 157 



which is closely pressed against the ventral surface of the 

 female in copulation, and which serves as a ready means 

 of distinguishing the sex. 



20. Histology. In all fundamental respects, the 

 tissues of the frog resemble those of the rabbit, but there 

 are certain differences of detail which it will be well to 

 mention here. The epidermis consists of two cell-layers only 

 an inner living layer and an outer horny one : the latter 

 is shed as a whole periodically, instead of bit by bit and 

 constantly, as is the rabbit's. From the inner layer are 

 developed abundant large, flask-shaped, simple glands (cf. 

 fig. 19[2]), which are imbedded in the 

 dermis. The frog's bones have very little 

 development of Haversian systems the 

 osteoblasts being arranged for the most 

 part in only one series of concentric 

 rings. In the striated muscle we find 

 the nuclei not confined to the superficial 

 region, but at all depths. In the blood 

 (fig. 79) the red corpuscles are much 

 larger than those of the rabbit, and oval 

 in shape, the central portion being thicker 

 than the margin, and they have a nucleus ; CORPUSCLES OF FROG. 

 the white corpuscles are much the same Diaw s t g, r sa e scale 

 as in the rabbit. The nerve-cells in the 

 frog's sympathetic ganglia have long been described as a 

 peculiar form of bipolar cell, which gives off two axis- 

 cylinders, one of which winds spirally round the other. 

 It has now been proved that they are unipolar, and that 

 the spiral axis-cylinder is efferent from a nerve-cell in the 

 spinal cord. This close winding of its termination round 

 the sympathetic nerve -cell makes up for the absence of 

 dendrons in the latter. 



21. Classification. Frogs and their allies are sharply 

 marked off from reptiles and mammals on the one hand, 

 and from fishes on the other, by the fact that they breathe 

 by gills in the larval stage of their life, and by lungs in the 

 adult stage. With this distinction go various others that 



