SKELETON. a7 
the number of young at a birth. This method of formation explains 
the varying position of the mamme 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, 
Fic. 30.—Diagram of the skeletogenous tissue in the caudal region of a vertebrate. 
bu, blood-vessels; epmu, epaxial muscles; hs, horizontal partition; hymy, hypaxial muscles; 
msd, msv, dorsal and ventral median septa; mys, myosepta; x, spinal cord; mc, 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 16. First is the corium, immediately beneath the epidermis, 
