706 ANATOMY OF VERTEBRATES. 



the back and belly shows that the position necessary for the due 

 progressive motions is unnatural at this stage of growth. 



The toes on each of the four feet were completely formed, and 

 terminated by curved, conical, horny claws ; but the natatory 

 fold of membrane of the fore foot had not the same proportional 

 extent as in the adult, and the spur of the hind foot did not 

 project beyond its socket in either specimen. In the smaller one, 

 which was a male, it presented the form of an obtuse papilla; 



while in the larger specimen, al- 

 though a female, it was more plainly 

 developed and more pointed, fig. 602, 

 f. This circumstance is in exact ac- 

 cordance with the known laws of the 

 development of sexual distinctions, 

 especially of those of secondary im- 

 portance, such as beards, manes, 

 plumes, horns, tusks, spurs, c., 

 which do not avail in distinguishing 



Hind-foot and spur, young female Oriiitho- fl, p cpxPS till townrrh thp n priori nf 

 rhynchus ; mag. LXXVIII'. L1G P 6 



puberty. 



In the Echidna hystrix the mammary glands resemble in 

 structure and position those of the Ornithorhynchus : but the 

 ducts, when the gland is functionally developed, open into a 

 small tegumentary pouch, fig. 603, c. The gland, ib. , is of a 

 flattened, subelliptic form. The lobules converge toward the 

 mesial line, in their course to terminate in the fundus of the 

 pouch. Each lobe is a solid parenchymatous body ; the duct is 

 more directly continued from a canal which may be traced about 

 halfway toward the fundus of the lobule ; the canal gives off 

 numerous short branches from its circumference, which subdivide 

 and terminate in clusters of subspherical * acini ' or secerning 

 cellules. The structure is on the same general plan as that of 

 the mammary glands in higher Mammals, but the cellules are 

 proportionally larger. Each gland consists of about 100 long, 

 narrow, flattened lobes, obtusely rounded at their free ends ; 

 they are surrounded by a loose capsule of cellular tissue, and 

 lie between a thick ( panniculus carnosus,' adherent to the abdo- 

 minal integument, ib. d, and the e obliquus externus abdominis ' 

 muscle, on a plane exterior or f lateral ' to the pouch. On each 

 side of the abdominal integument, about two inches in advance 

 of the cloaca, and about three inches and a half from the 

 base of the tail, is the aperture, which is longitudinal and di- 

 rected towards the median line. The skin of the abdomen, where 



