{This Section missing from Second Edition.} -Section 11. It is not very improbable that the kidney of the frog shares, or performs, some of the functions of the rabbit's liver, or parallel duties, in addition to the simply excretory function. Since specialization of cells must be mainly the relatively excessive exaggeration of some one of the general properties of the undifferentiated cell, it is not a difficult thing to imagine a gradual transition, as we move from one organism to another, of the functions of glands and other cellular organs. It is probable that the mammalian kidney is, physiologically, a much less important (though still quite essential) organ than the structures which correspond to it in position and development in the lower vertebrate types.- Section 12. The lymphatic system is extensively developed in the frog, but, in the place of a complete system of distinctly organized vessels, there are great lymph sinuses (compare Section 1). In Figure 5, Sheet 12, the position of two lymph hearts (l.h., l.h.) which pump lymph into the adjacent veins, is shown. Section 13. The skull of the frog will repay a full treatment, and will be dealt with by itself later. The vertebral column (Sheet 12) consists of nine vertebrae, the centra of which have faces, not flat, but hollow in front (pro-coelous), and evidently without epiphyses (compare the Rabbit). The anterior is sometimes called the atlas, but it is evidently not the homologue of the atlas of the rabbit, since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal, and since, therefore, it is probable that the mammalian skull = the frog's skull + one (or more) vertebrae incorporated with it. Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod. The vertebrae have transverse processes, but no ribs. Section 14. The fore-limb (Figure 6, Sheet 12) consists of an upper segment of one bone, the humerus, as in the rabbit; a middle section, the radius and ulna, fused here into one bone, and not, as in the mammalian type, separable; of a carpus, and of five digits, of which the fourth is the longest. The shoulder girdle is more important and complete than that of the higher type. There is a scapula (sc.) with an unossified cartilaginous supra-scapula (s.sc.); the anterior border of the scapula answers to the acromion. On the ventral side a cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in this type), runs to the sternum, and answers to the clavicle of the rabbit. In the place of the rabbit's coracoid process, is a coracoid bone (co.), which reaches from the glenoid cavity to the sternum; it is hidden on the right side of Figure 6, which is a dorsal view of the shoulder girdle. There is a pre-omosternum (o.st.) and a post-omosternum, sometimes termed a xiphisternum (x.).