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NORTH AMERICAN FAUNA 74 



Fig. 113. Histological stages in the annual cycle of the seminiferous epithelium in 

 testes of Pacific walruses. Associations: A, B, C, recrudescence; D, E, F, spermiogenesis; 

 and G, H, retrogression. (Photos by F. H. Fay) 



Association A. — Tubule diameter 114-146 fim; basement membrane 2-3 fim 

 thick; epithelium 50-60 /^m tall and composed of Sertoli cells and spermatogonia 

 only. Initially, the spermatogonia are in a single cell layer adjacent to the 

 basement membrane, and the chromatin granules of their nuclei are about 

 intermediate in size between those of types A and B. In later stages, the nuclei are 

 darkly stained and the cells more numerous, forming a generative epithelium 2 

 to 4 cells thick. This change could be due to proliferation of the "dormant" 

 spermatogonia or, more likely, to contraction of the tubules associated with the 

 general reduction in volume of the testis, for I saw no evidence of mitosis. 



Association B.— Tubule diameter 76-148 fim; basement membrane 2-3 ixm; 

 epithelium 40-60 fim tall; spermatogonia nearly all type B and in several layers, 

 later subsiding to a single layer with a few mitotic figures. Some primary and 

 secondary spermatocytes present, but no spermatids; nuclei of the Sertoli cells 

 becoming indistinct. 



