SKELETON. 



37 



the number of young at a birth. This method of formation explains 

 the varying position of the mammae and also the occasional occurrence of 

 more than the normal number (polymastism) in man and other mam- 

 mals. Each gland is provided with a nipple and of these there are two 

 kinds (fig. 29). In the one the whole surface on which the lacteal 

 ducts empty becomes elevated, in the other the region around the 

 openings of the ducts becomes drawn out into a tube with the ducts 

 opening at the bottom (ungulates). 



THE SKELETON. 



The term skeleton as used here is applied to any of the harder parts 

 of the body, developed from the mesoderm and serving for support, 



FIG. 30. Diagram of the skeletogenous tissue in the caudal region of a vertebrate. 

 bv, blood-vessels; epmu, epaxial muscles; hs, horizontal partition; hymy, hypaxial muscles; 

 msd, msv, dorsal and ventral median septa; mys, myosepta; n, spinal cord; nc, notochord. 



for the attachment of muscles, for protection and the like. This ex- 

 cludes any purely epidermal hard parts, and these have been included 

 with the integument. 



As the skeleton can only develop where there is mesenchyme, the 

 distribution of the chief skeletogenous parts, sometimes called the 

 membranous skeleton, may be given here, continuing the account from 

 page 1 6. First is the corium, immediately beneath the epidermis, 



