422 HISTOLOGY 



sexually at any time during the adult life of the organism. This is true 

 of most plants whose growing cells are also their potential reproductive 

 cells, which, when the proper conditions occur, will mature into male 

 and female gametes. In this case we find an organism which develops 

 its gonads anew at each reproductive period of its life. The same facts 

 are true of many sponges and coelenterates and other low animals. 



As a first step in the differentiation and higher organization of such 

 plants and low animals we find that one or more of the somatic tissues 

 lose the reproductive power. For instance, all of the tissues of a be- 

 gonia plant, as stem and roots and even leaves, may be propagated to 

 form a new plant which will produce reproductive cells. This power is 

 lost to the leaves hi a hickory tree, while the roots and stem retain it; in 

 a hyacinth and some lilies and iris, the roots and leaves have both lost it 

 and the power is confined to certain parts of the root-stock and the 

 flower. It may even be lost to the flower in some highly cultivated 

 plants that bear no fertile seeds. 



The preceding paragraphs will themselves serve to harmonize to a 

 large degree the view they express with a second view regarding the 

 origin of the reproductive cells : that they are not cells of a separate line 

 of descent, retaining powers that the somatic cells have lost, but that 

 they may arise from the body tissues (usually described as mesodermal 

 tissues) by processes of differentiation similar to those of any of the 

 other tissue cells in the various 'tissues. These supposedly antagonistic 

 cases merely show a later somatic differentiation: that the final differ- 

 entiation may never occur, or it may be early, or it may be deferred until 

 a comparatively late period. 



As to the time at which sex is determined there appears to be more 

 doubt and wider divergence of views as time passes and investigation 

 along new and old lines proceeds. The idea that a high degree of nour- 

 ishment during early embryonic life resulted in a majority of females 

 and a low degree in more males has been shown to mostly mean a prior 

 killing off of young females by starvation. Statistical methods and 

 experiment along other lines have failed to throw light on the matter. 

 Our chief hope of definite knowledge of more immediate causes seems 

 to lie at present in certain cytological investigations on insects. This 

 will be brought out in the following parts. 



LITERATURE 



WILSON, E. B. " The Cell in Development and Inheritance." New York, 1900. 



