The American journal of anatomy . dichotomously or otherwise. From the various figures itwill 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 divideinto 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 fromglands at birth, only two distinct anastomoses occur (fig. 3).However, others hav
The American journal of anatomy . dichotomously or otherwise. From the various figures itwill 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 divideinto 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 fromglands at birth, only two distinct anastomoses occur (fig. 3).However, others have been observed in cleared preparation atthe same stage. It will be noted that along the secondary and tertiary ductsnumerous lateral l)uds occur (figs. 3 to 6). Many more of themare present on the distal than on the proximal ducts. Such budslater form collateral branches destined to develop into ductssimilar to those already present. This point will be more clearlybrought out in the older stages. STUDIES ON THE MAMMARY GLAND 365. Fiji;. 0 Internal view of a wax model reconstructed from tiie left seeond iu-cuinal <y\:\ut\ of a newborn all)ino rat. X 40. (For letterinji, see fio;. o.) 866 J. A. MYERS A point which has been discussed at some length recently andone which has proved to be of considerable importance in experi-mental work is that of the variation in the relative size and de-velopment of the various glands in the same individual, and ofglands from different individuals of the same age. Lane-Clay-pon and Starling (06) in working on the growth and activity ofthe mammary gland concluded hyperplasia of preg-nancy is caused by chemical substances formed in the substances passing through the placenta into the maternalblood-stream cause growth of the mammary gland. To decidedefinitely as to just what tissues cause this growth Lane-Clayponand Starling injected extracts of placenta, placenta and uterus,ovaries, fetus, fetus together with the placenta
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