PHYSIOLOGY, CHEMISTRY AND PATHOLOGY 569 



Many authors consider the interstitial cells of the ovary and those of 

 the testis as belonging to the same category. But the evidence in favor 

 of this view is very unsatisfactory. The cells of the ovary are, at any 

 rate, much more conspicuous than those of the testis, and the histological 

 differences between the two are sufficient to warrant considerable hesi- 

 tation in forcing an analogy. 



At the outset we are met with the perplexing fact that the inter- 

 stitial cells are not present in all species. Out of more than one hundred 

 species so far examined, fifty per cent are said to possess none. It is 

 true that some writers are inclined to believe that some trace of the in- 

 terstitial tissue is always present, though the cells may be very incon- 

 spicuous and may be overlooked, owing to the fact that under certain 

 conditions they do not react to ordinary staining reagents. Again, it has 

 been supposed that the cells undergo periodic changes of such a character 

 that at certain times they may vary, being at one time more, and at other 

 times less abundant or conspicuous. The subject of the occurrence or 

 non-occurrence of the interstitial cells in the various species of mammals 

 still requires careful investigation. 



The morphological aspects of the question are dealt with in another 

 part of the present work, but it will be useful to mention some papers of 

 recent date from which most of the literature can be collected. In 1916 

 O'Donoghue gave a good account of the interstitial cells in marsupials. 

 Rasmussen (b) (1918) describes the cyclical changes in these cells in 

 Marmola monax and gives a full account of the literature. Athias (e) in 

 1.919 has published a careful study of the interstitial cells in the cheiroptera. 



There seems to be no doubt that th& interstitial cells undergo cyclical 

 changes of such a nature that they reach their greatest development during 

 pregnancy and lactation. 



As to the origin of the cells in question, there has been much con- 

 troversy. The history of this will be found in the papers referred to 

 above. Two sources have been urged specially: (1) from the stroma 

 (connective tissue) either directly or indirectly through the theca interna 

 of atretic follicles, (2) from epithelial tissue (germinal epithelium). The 

 majority of writers have favored the former view, but the evidence is by 

 no means convincing, and histologists will be reluctant to admit the con- 

 nective tissue origin of cells possessing so glandular an appearance. More- 

 over, even if it be certain that the interstitial cells are directly derived 

 from some of the "stroma" cells, it is by no means clear that these par- 

 ticular elements are themselves of connective tissue origin. Some epi- 

 thelial structures may easily have become involved in the stroma at an 

 earlier stage. The origin of the stroma cannot be alleged to have been 

 settled in all its details. In any case the interstitial cells are now gen- 

 erally regarded as a tissue sui generis. 



The fully developed interstitial tissue presents all the appearances of 



