354 J. A. MYERS 



gland can be obtained in a comparatively short time. The vari- 

 ous developmental stages are therefore easily obtained and 

 controlled. 



In the present study three methods were used. (1) Micro- 

 scopic sections were studied ; (2) wax reconstructions (of the new- 

 born) were made according to Born's method; (3) the ducts were 

 studied from cleared preparations. These preparations were 

 made according to the method employed by Lane-Claypon and 

 Starling ('06). The skin of the entire ventral part of the body 

 was removed, spread out on a sheet of cork, and fixed in a mer- 

 curic chloride-formalin solution (10 per cent formalin in a sat- 

 urated aqueous solution of mercuric chloride). The corium and 

 tela subcutanea containing the gland were then removed in a 

 single sheet. In the older specimens it was usually necessary 

 to dissolve out the fat with alcohol and ether before staining. 

 The preparations were then placed in a very dilute solution of 

 alum-hematoxylin or carmalum until they were sufficiently 

 stained. When necessary, the excess stain was washed out with 

 acid-alcohol. After dehydration, beechwood creosote and cedar 

 oil were used as clearing agents, the specimens being first placed 

 in creosote for a few hours and then transferred to cedar oil. 

 After being thoroughly cleared they were mounted in damar 

 on glass slides. 



Frank and Unger ('11) state that Starling's technique of 

 staining and clearing the breasts, as in the rabbit, could not be 

 employed in the rat. No explanation is given as to why Star- 

 ling's method could not be employed. It is true that this method 

 is rendered quite difficult owing to the development of the pan- 

 niculus carnosus muscle in the thoracic region. Being very 

 closely related to the milk-ducts, it is almost impossible to dis- 

 sect this muscle off without destroying some of the mammary 

 gland. However, good cleared preparations can be studied to 

 advantage even when the muscle is intact. The fact that the 

 panniculus carnosus muscle is lacking in the abdominal and in- 

 guinal regions makes the study of the glands in these regions com- 

 paratively easy. Fat is quite easily removed in cleared prepa- 

 rations, hence the considerable quantities of it deposited in the 



