202 



THE EVOLUTION OF MAN. 



plugs ( 2 , 3 ), either owing to the softening and breaking 

 up of the central cells, or as the result of a fluid internally 

 secreted. Some of these skin-glands remain unbranched, as, 

 for instance, the sweat-glands (e,f,g). These glands, which 

 secrete the sweat, are of great length, their ends forming a 

 coil ; they never branch, however ; and the same is to be 

 said of the glands which secrete the fatty wax of the ears. 



Fig. 214. — Rudiments of tear-glands 

 from a human embryo of four months. 

 (After Koelliker.) 1. Earliest rudiment the 

 shape of a simple, solid plug. 2 and 3. Fur- 

 ther developed rudiments, which branch 

 and become hollow : a, a. solid offshoot ; 

 e, cell-covering of the hollow offshoot ; /, 

 rudiment of the fibrons covering, which 

 afterwards forms the leather-skin round 

 the glands. 



Most other skin-glands give out 

 shoots and branches, as, for in- 

 stance, the tear-glands, situated 

 on the upper eyelid, which secrete 

 the tears (Fig. 214), and also the 

 sebaceous glands, which produce the fatty sebaceous matter, 

 and generally open, into the hair-follicles. The sweat 

 and sebaceous glands occur only in Mammals. The tear- 

 glands, on the contrary, are found in all the three classes of 

 Amnion Animals, in Reptiles, Birds, and Mammals. They 

 are not represented in the lower Vertebrates. 



Very remarkable skin-glands, found in all Mammals, 

 and in them exclusively, are the milk-glands (glandulce 

 mammales, Figs. 215, 216). They supply milk for the 

 nourishment of the new-born Mammal. Notwithstanding 



