336 Papers from the Department of Marine Biology. 



of migration on the basis of germ-cell counts. The fact that at 

 progressively later stages in development the germ-cells were found 

 in the entoderm near the lateral border of the area pellucida, then in 

 the visceral plate of the lateral mesoderm and more medially in the 

 entoderm (later also in the closed gut and enveloping mesenchyma), 

 then in the mesentery, and finally across the ccelomic angle among the 

 peritoneal epithelium of the sexual gland (genital fold), indicates that 

 this represented the actual route of migration. The further facts that 

 only an occasional germ-cell was seen in mitosis throughout the series 

 (second to thirty-second day), that only a few were seen in conspicuous 

 stages of degeneration, and that practically none appeared in blood- 

 vessels (and only a few in the periaortic mesenchyma) seems to prove 

 the indicated route as the actual and substantially exclusive migra- 

 tion path. In the case of Cymatogaster, also, Eigenmann found that 

 the sex-cells do not divide from the period before the first somites 

 have formed at which time they are first distinguishable up to 

 about the period when the young fish reaches a length of 7 mm. 



The close similarity, which in early stages amounts practically to an 

 identity, between the primordial germ-cells and the cells of the ento- 

 derm (especially the yolk-sac entoderm) need not, and almost certainly 

 does not, imply a derivation of germ-cells from entoderm-cells. In the 

 first place, the germ-cells can always be distinguished from the ento- 

 dermal cells (in spite of structural and tinctorial similarity between the 

 cells) by the rounded (spherical or oval) form of the germ-cells. The 

 cells of the yolk-sac entoderm, which they resemble most closely, are of 

 irregular shape. Occasionally a germ-cell may become wedged in 

 between the entoderm-cells and so forced into very intimate cyto- 

 plasmic relationship and into irregular form, in which condition it may 

 be impossible to distinguish between the two; but the resemblance 

 signifies simply, most probably, a similar low grade of differentiation 

 from the original blastomeres. The germ-cells are therefore not 

 derived from the entoderm-cells by differentiation. These two types 

 of cells are similar because they are practically similarly undifferen- 

 tiated, both containing a large, finely granular, pale nucleus and a large 

 cytoplasmic content of yolk-globules. The germ-cells simply remain 

 dispersed among the entodermal-cells after the segmentation stages and 

 from here migrate via the area pellucida to the mesentery and thence 

 to the gonads. The germ-cells here, as in certain invertebrates 

 (e. g., Ascaris, Boveri) most probably have been held apart from the 

 soma-cells since an early segmentation stage and then, from widely 

 scattered areas, have migrated through progressively more sharply 

 segregated foci to the gonads, maintaining thus a continuous germ-cell 

 cycle from blastomeres (perhaps a particular portion of the fertilized 

 egg, as, for example, the pole disk of certain Diptera, Hegner) to the 

 gonocytes of the definitive genital glands. 



