372 J. A. MYERS 



Eight weeks. In the eight-weeks spechnens, the first and sec- 

 ond thoracic glands have increased to such an extent that their 

 ducts overlap. This overlapping may occur on one side only, 

 there being an interval between the corresponding ducts of the 

 opposite side. Concerning the other glands of this stage, very 

 little need be said except that the ducts have increased in length 

 and new branches have been added. It should be pointed out 

 that at this stage the rat possesses only four distinct masses of 

 mammary gland tissue. All of the thoracic glands of each side 

 have their ducts so interlaced as to form one apparently solid 

 mass of ducts. Also the abdominal and inguinal glands of each 

 side are so matted together that no dividing line exists between 

 them. 



Nine and ten weeks. The nine and the ten-weeks stages (see 

 fig. 13) show a tremendous increase in development over the 

 previously described stages. The ducts have spread out to cover 

 a much larger area. Not only is the overlapping of ducts more 

 complicated but each gland has produced a large number of 

 medial and lateral branches. For example, some of the medial 

 ducts of the first thoracic glands of each side have grown so 

 near to the mid-line that only a narrow space separates them. 

 In the case of the first inguinal glands the medial ducts actually 

 reach the mid-line, and a slight overlapping of the ducts of oppo- 

 site sides occurs. The medial ducts of the second inguinal glands 

 almost surround the vagina, the ducts from the opposite sides 

 very nearly meeting in the mid-line both cephalad and caudad to 

 the vagina. At this stage the proximal or secondary ducts also 

 bear large numbers of short collateral ducts which have devel- 

 oped from the lateral buds mentioned in the earlier stages. 



It is frequently stated that from birth to puberty the human 

 milk-ducts undergo very little development, merely keeping pace 

 with the general body-growth. At puberty an abrupt change 

 like that affecting the entire organism is said to occur. So far 

 as the rat is concerned, an examination of figures 2 and 7 to 13 

 will show that in the virgin, from birth to the age of ten weeks, 

 there are apparently two periods when the increase in the mam- 

 mary gland ducts is somewhat marked. The first period occurs 



