STUDIES ON THE MAMMARY GLAND 367 



there is a decided change in the structure and size of the mam- 

 mary glands of Dasyurus viverrinus when ovulation is not suc- 

 ceeded by pregnancy. 



A comparison of the individual glands of the rat at birth and 

 at two weeks (figs. 2 to ()) will show that there is consideral^le dif- 

 ference in the size and develoi^ment of the various glands in the 

 same rat, sometimes even in the same pair of glands (fig. 2). 

 It has also been observed that corresponding glands from differ- 

 ent rats of the same age and approximately equal weights show 

 considerable variation in size and complexity of structure. The 

 differences in size and de\'elopment observed by me in the rat 

 are not so marked as those described by Frank and Unger, 

 Bouin and Ancel, and O'Donoghue. Yet they are worthy of 

 mention and are certainly sufficient to prove that the normal 

 structure and variability under different conditions of any part 

 of the animal body should be thoroughly investigated before 

 conclusions are drawn from experimental work. It is quite pos- 

 sible that such knowledge of the mammary gland of the rabbit 

 would have changed decidedly the conclusions of Lane-Claypon 

 and Starling. 



2. Growth of the ducts 



In the newborn rat, models wxre reconstructed showing one 

 gland of each of the six pairs (figs. 3 to 6). At two weeks, all 

 the glands are represented in figure 2, to show the general topog- 

 raphy of the ducts. At the other stages (1 week, 2, 3, 4, 5, 7 

 and 9 weeks, figs. 7 to 13) it is found unnecessary to reproduce 

 all the glands, so only the abdominal and inguinal glands of the 

 left side are shown. A general description of all glands at each 

 stage will be given, however. 



Neivboni. The figures drawn from wax reconstructions of the 

 glands at birth (figs. 3 to (5) show that with few exceptions the 

 ducts of each gland in these stages all lie approximately in the 

 same plane, parallel to the surface of the skin. It was noticed 

 that in regions where there are no obstructions the ducts spread 

 very freely and cover a considerable area. The unobstructed 

 ducts almost invariably lie in a single plane at this stage of 



