The developmeat of the urinogenital organs of the lamprey. 75 



extent of denying the existence of metamerism in the sex-cells of the 

 Selachians, 



When we consider in a general way the whole subject of the 

 primitive sex-cells in Vertebrates, two questions may be asked. First, 

 is it a fact that these elements have a metameric arrangement in the 

 embryos of any existing Gnathostomes and second, is the use of 

 the term gonotom justifiable in phylogenetic speculation, even if no 

 such arrangement can be recognized? To the former question I 

 should give a negative, to the latter an affirmative answer. 



First, taking into consideration the reproductive cells in the Am- 

 phibia {Bombinator) as described by Goette (1878), in Petromyzon 

 (GoETTE and myself), Selachians (Rückert and Miss Gregory) and 

 in some Teleostei {Cymatogaster e. g. as described by Eigenmann 

 in a series of interesting papers, 1891, 1894 and 1897), we observe 

 that the sex-cells can be recognized before or during the formation 

 of the middle germ-layer. In Cymatogaster according to Eigenmann 

 (1897, p. 166): "The sex-cells are segregated very early, before any 

 protovertebrsB are formed. Judging from their size they are seg- 

 mentation cells of the fifth generation or there abouts." In Petro- 

 myzon, as I have shown, the sex- cells can be recognized in the ento- 

 derm before the mesoderm is completely formed, and only later come 

 to lie in the middle germ-layer. They are certainly recognizable at 

 a time and in a position that fail to support the view that they are 

 metameric. In Urodeles {AmUystoma, Diemyctylus and Spelerpes), 

 as I have myself observed, the conditions are very similar to those 

 in Anura and Petromyzon. In the majority, if not in all, of the forms 

 above mentioned the yolk is retained in the sex-cells long after it 

 has disappeared from the other cells of the embryo. It is this long 

 retention of the yolk together with their larger size which enables 

 one to recognize the sex- cells at so early a stage, and has led Goette 

 (1891) to make the following generalization : "es wird dadurch meine 

 schon bei jener frühern Gelegenheit (1878) nachdrücklich verfochtene 

 Ansicht unterstützt, dass die Vererbung nicht vom fertigen 

 Mutterthier aus auf die erst in ihm entstandenen Keime, 

 sondern von einem Keime direct auf die nächste Gene- 

 ration von Keimen erfolge." Whatever be the truth of this 

 generalization, it is obvious that certain more modest conclusions 

 seem to follow more immediately and naturally from the data above 

 recorded. It is obvious that when we speak of the early differentiation 

 of the sex- or germ-cells, we really mean the early differentiation of 



