1()9 



duces the parent and the ovum which produces the offspring the character 

 of the original ovum is never lost by differentiation. There is then a con- 

 tinuous chain of reproductive cells quite apart from the body cells or 

 more frequently a series of body cells through which the unchanged germ 

 plasm of the parent is transmitted to future generations. The germ cells 

 are, therefore, not the product of the adult Ijody but the direct offspring 

 from the germ cell of the preceding generations. 



The observations bearing out much of this theory have been mostly con- 

 fined to invertebrates. All of our works on the comparative anatomy of 

 vertebrates, as well as our works on embryology, tell us that the sexual or- 

 gans in vertebrates arise from the germinal epithelium which is not differ- 

 entiated until the embryo is completely formed. The most lucid descrip- 

 tions of the early stages were given by Balfour for Elasmobranchs ten 

 years -ago, and the latest observations published by Jungersen in 1889 have 

 not given anything concerning the stages less than two millimeters long. 



While preparing the sections for the ontogeny of C>imatogaster aggr'egaius, 

 one 'of the viviparous Embiotocidae, I frequently observed large, indifferent 

 cells in the mesoblast. I at first supposed them to be cells in a pathologi- 

 cal condition. When, however, all the eggs from one ovary were observed 

 to contain such cells, I re-examined every embryo, and soon found that the 

 cells are not pathological, but are a normal structure present in all embryos 

 of a certain age. Further study showed them to be sex-cells of the future 

 germinal epithelium. Our knowledge of the early stages of the sex-cells 

 of vertebrates does not extend back beyond the condition described by 

 Balfour and .]ungersen. In the present study I have been able to trace 

 them back to probably the fifth segmentation. 



Our knowledge of the sex-cells in general has been summed up by Weis- 

 mann as follows : " In certain insects the development of the egg into the 

 embryo, that is, the segmentation of the egg, begins with the separation of 

 a few small cells from the main body of the egg. These are the reproductive 

 cells, and at a later period they ai"e taken into the interior of the animal 

 and form its reproductive organs. Again, in certain smaller fresh-water 

 Crustacea (Daphnidte) the future reproductive cells become distinct at a 

 very early period, although not quite at the beginning of segmentation, i. e, 

 when the egg has divided into not more than thirty segments. Here also 

 the cells which are separated early form the reproductive organs of the 

 animal. The separation of the reproductive cells from those of the body 

 takes place at a still later period, viz. at the close of segmentation, in Sag- 



