1 62 BIOLOGY OF THE LABORATORY MOUSE 



dose proximity to capillaries. The nuclei of these fibroblasts become 

 rounded and the cytoplasm increases in amount. Gradually fat accumu- 

 lates in the cytoplasm and the nucleus is pushed to the periphery. The cells 

 increase immensely in size during this change and adipose cells rapidly 

 rebuild the fat pads. The collapsed alveoli form irregular clumps of cells 

 which gradually undergo further degeneration. In the completely regressed, 

 resting gland the lumina of the ducts are narrow, the epithelial cells lining 

 them are small and darkly staining. The connective tissue sheath surround- 

 ing the ducts is increased in thickness. The glands remain in resting condi- 



FiG. 87. — Mammary gland at resting stage. (Xioo.) 



tion until the following pregnancy, when the described changes are repeated 

 (Fig. 87). 



In old females the glands undergo gradual involution. Part of each duct 

 system atrophies and only the main ducts and a few secondary branches 

 remain. The connective tissue surrounding the ducts becomes less cellular 

 and more homogeneous. 



BIBLIOGRAPHY 



1. Agduhr, E. 1927. Studies on the structure and development of the bursa 

 ovarica and the tuba uterina in the mouse. Acta. Zool. 8: i-i 33. 



2. Allen, Edgar. 1922. The oestrous cycle in the mouse. Am. J. Anat. 30: 297- 

 348. 



3. Allen, Edgar (editor). 1939. Sex and internal secretions. WiUiams and 

 Wilkins Co., Baltimore. 



4. Allen, Ezra. 1918. Studies on cell division in the albino rat. III. Spermato- 

 genesis: the origin of the first spermatocytes and the organization of the chromo- 

 somes including the accessory. J. Morph. 31: 133-187. 



5. Allen, Ezra. 1940. Variations in the accessory chromosomes in the metaphase 

 and anaphase of the first spermatocytes of the albino rat. J. Morph. 66: 453-462. 



6. Benoit, J. 1926. Recherches anatomiques, cytologiques et histo-physiologiques 

 sur les voies excretrices du testicule chez les mammif^res. Arch, anat., hist.et 

 embr. 5: 175-412. 



