364 J. A. MYERS 



As compared with the primary duct, the secondary ducts pre- 

 sent a rather extensive course, after which they break up each 

 into two or more branches (tertiary ducts). It will be noticed 

 that at birth (figs. 3 to 6) the terminal branches of each tertiary 

 duct vary from one to three in number. On the end of most 

 terminal branches is a small bud-like enlargement. These en- 

 largements were described as true alveoli by earlier investigators, 

 but this was found later to be incorrect. Billroth (according to 

 Berka '11) doubts whether completely formed end-vesicles occur 

 in young human virgins. While he called the terminal enlarge- 

 ments 'real end-vesicles,' yet he adds that they later develop 

 into ^true end-vesicles' and further multiply during pregnancy. 

 Berka ('11) states that true alveoli do not occur in young (human) 

 virgins. Similarly the terminal enlargements found on the milk- 

 ducts of young rats are not true alveoli, but are only enlarged 

 growing processes corresponding to the end-buds found in other 

 developing glands. The microscopic structure of these enlarge- 

 ments and the development of true alveoli will be discussed in 

 a later paper dealing with the histology of the mammary gland. 



The question often arises as to whether the ducts of glands 

 branch dichotomously or otherwise. From the various figures it 

 will be seen that the more proximal parts of the terminal seg- 

 ments usually follow the dichotomous method, but the distal por- 

 tions, as stated above, may terminate as a single duct or divide 

 into two or three branches. In the last thoracic gland (fig. 4) 

 the secondary branches approach true dichotomous division. 



Anastomoses occur between ducts, but they are not very fre- 

 (juent in the newborn rat. In the reconstructions made from 

 glands at birth, only two distinct anastomoses occur (fig. 3). 

 However, others have been observed in cleared preparation at 

 the same stage. 



It will be noted that along the secondary and tertiary ducts 

 numerous lateral l)uds occur (figs. 3 to 6). Many more of them 

 are present on the distal than on the proximal ducts. Such buds 

 later form collateral branches destined to develop into ducts 

 similar to those already present. This point will be more clearly 

 brought out in the older stages. 



