MAMMALIAN SEXUAL CHARACTERS 135 



epithelium. In the earliest Mammals we may 

 suppose that the young were born in a well-de- 

 veloped condition, for at first the supply of milk 

 would not have been enough to sustain them for a 

 long time as their only food. We must also suppose 

 that the mother began to cherish the young, keeping 

 them in contact with her abdomen. Then being 

 hungry they began to suck at her hair or fur. The 

 actual development of the milk glands in Marsupials 

 has been described by Bresslau 1 and by O'Donoghue. 2 

 The rudiment of the teat is a depression or invagina- 

 tion of the epidermis from the bottom of which six 

 stout hairs arise. The follicles of these hairs extend 

 down into the derma, and from the upper end of the 

 follicle, i.e. near the aperture of the invagination, a 

 long cellular outgrowth extends down into the derma, 

 branches at its end, and becomes hollow. These 

 branches are the tubules of the future milk gland. 

 Another outgrowth from the follicle forms a sebaceous 

 gland. Later on the hairs and the sebaceous glands 

 entirely disappear, and the milk gland alone is left 

 with its tubules and ducts opening into the cavity 

 of the teat. This is clear evidence that the milk 

 gland was evolved in connexion with hairs, and was 

 an enlargement of glands opening into the hair 

 follicle, but it is difficult to understand why a seba- 

 ceous gland is developed and afterwards disappears. 

 This would seem to indicate that the milk gland was 

 not a hypertrophied sebaceous gland, but a distinct 

 outgrowth, which however had nothing to do with 

 sweat glands. 



That the intra-uterine gestation, or its cessation, 

 were not originally necessary to determine the 



1 Stuttgart, 1901. Q.J.M.S., Ivii., 1911-12. 



