456 ROBERT W. HEGNER 



of secondary somatic differentiation (the Sertoli cells representing 

 the soma of the testis) ; the first somatic differentiation occurring 

 when the tissue cells become differentiated from the germ cells 

 in the embryo. 



It may be worth mentioning at this place that among the 

 invertebrates, instances of changes which take place in the germ 

 cells of the male are known which may be brought into line with 

 the facts in the history of the Sertoli cells of man. For example, 

 in the spermatogenesis of the parasitic copepod,Laemargus muri- 

 catus, McClendon ('06) found that the cells in some of the groups 

 of four spermatids became filled with an achromatic substance 

 which resembled yolk — a substance called 'Austreibestoff' by 

 Heider ('79). The origin of this substance could not be deter- 

 mined, but it probably came from the nucleus. The cells thus 

 affected serve as nurse cells for the spermatozoa. 



5. THE KEIMBAHN IN OTHER ANIMALS 



It is not possible in this place to give either a general account 

 or detailed account of the Keimbahn as it has been described in 

 groups such as the Porifera, Coelenterata, etc., since we are 

 here interested especially in the peculiar substances which appar- 

 ently determine the Keimbahn, and thus far no results of impor- 

 tance have been obtained from studies of these animals. Under 

 the above heading, however, I wish to mention a few stages in 

 the development of certain forms, widely separated in the animal 

 kingdom, which have either been compared or can be compared 

 with conditions such as we have described in the preceding 

 portions of this paper. 



Metanudeoli. Haecker's ('97) belief that the 'Aussenkorn- 

 chen' in Cyclops are of nucleolar origin and Silvestri's ('06, '08) 

 discoveries in parasite Hymenoptera indicate that in certain 

 instances the nucleoli may play some role in the differentiation 

 of the primordial germ cells. 



The large nucleolus in the germinal vesicle of the medusa, 

 Aequorea forskalea (fig. 26, A), according to Haecker ('92), 

 disappears from the germinal vesicle about half an hour after the 

 egg is laid, and a similar body becomes evident near the egg 



