566 Gideon S. Dodds. 



b. Gonotome theory. The advocates of this view agree with 

 those who hold a peritoneal origin, that the primordial ova are 

 actually in the line of the functional sex cells. They contend 

 however that they do not arise by the transformation of peri- 

 toneal cells, nor indeed in the peritoneum at all, but from a defin- 

 ite part of the segmental mesoblast of the embryo, whence they 

 are brought into the peritoneum during the growth of the embryo. 

 This view was first advocated by Riickert ('88) in a study of 

 Pristiurus, and was given fuller expression by Van Wijhe ('89) 

 in studies in Scyllium and Pristiurus. The later writer applied 

 the term 'gonotome' to the part of the segment producing 

 the sex cells. He considered that segmentation extends later- 

 ally a short distance into the lateral plate, and to this part of it 

 he applied the term 'hypomere.' The hypomere is equivalent 

 to the gonotome. Hall ('04) applied this theory to Amblystoma, 

 and Dustin ('07) to three other Amphibia; Triton alpestris, Rana 

 fusca, and Bufo vulgaris. He also describes a transformation 

 of peritoneal cells at a later developmental period and makes these 

 two lines of germ-cells the basis of interesting phylogenetic spec- 

 ulation. 



c. Early segregation. The advocates of this view agree with 

 the two views just presented, that the primordial ova are true 

 germ-cells but consider that neither of the preceding explanations 

 of their origin are correct. They hold that the germ-cells are early 

 segregated, during cleavage stages, before organs or germ-layers 

 are formed, and later migrate into the coelomic epithelium where 

 they are readily recognized. According to this view they are not 

 transformation products, nor do they arise from or belong to 

 any germ layer, but they are primitive cells early set apart. 

 Careful investigations upon vertebrates of every group except 

 mammaUa have shown that the germ-cells may be definitely 

 recognized in very early stages of embryonic development, and 

 that during the course of embryo formation they migrate into the 

 position of the gonads. The following list gives the writers who 

 have found evidence of an early segregation of the germ-cells 

 of vertebrates : 



