368 J. A. MYERS 



growth. The last thoracic and the abdominal glands (fig. 4) are 

 good examples. Here, as is readily seen, the integument is free 

 from appendages or anything that would tend to limit the uni- 

 form spreading of the ducts. It was also noticed that in regions 

 where there are obstructions the branching of the ducts is more 

 irregular and that the ducts arrange themselves so as to lie in 

 more than one plane. The best example of this condition is 

 seen in the last inguinal mammary gland (fig. 6). Here as pre- 

 viously stated the nipple lies in the caudal part of the inguinal 

 region. To the lateral side of this gland the area available for 

 ramification of ducts is obstructed by the hind-limb while to the 

 medial side the external urinary and genital organs limit the 

 area. This leaves only a very narrow region free for the distri- 

 bution of ducts. Consequently, instead of spreading freely and 

 occupying a single plane, the ducts branch so as to lie in three or 

 four planes, each of which is parallel to the surface. Small areas 

 where the growth of the mammary gland is obstructed by lym- 

 phatic glands are shown in several of the figures of later stages. 

 Thus the course, branching, and spreading of the ducts depend 

 largely upon the available space. 



The reconstructions show that the ducts of the abdominal and 

 inguinal glands lie much deeper from the surface than those of 

 the thoracic glands. This is probably due to the absence of the 

 panniculus carnosus muscle which may to some extent prevent 

 the ducts from passing deeply in the thoracic region. Also con- 

 siderable fat is present in the abdominal and inguinal region, 

 which is apparently a very favorable substance for the ramifica- 

 tion of ducts. 



First week. At the end of the first week the ducts are not 

 much different from those at birth except that they are slightly 

 more branched. The ducts of the first thoracic glands give off 

 a few branches which take a caudal direction; all the other 

 branches of this pair of glands pass cephalad. The second pair 

 of glands sends the ducts in a latero-cephalic direction, but 

 many collaterals are given off some of which take a cephalic while 

 others take a caudal direction. The third thoracic glands send 

 their ducts in the same general direction as the second. The 



