36 PETER OKKELBERG 



ameboid forms have been observed in the yolk-laden germ cells 

 of the early embryo, and it is not likely that they are capable of 

 independent movement, but it is probable that they are trans- 

 ported to the germ-gland region by a shifting of the tissues dur- 

 ing somatic differentiation. In later stages, after all the yolk 

 has disappeared, the germ cells extend farther and farther craniad. 

 In sections of these stages the individual germ cells always have 

 a spherical shape and no pseudopodial processes have been seen 

 to indicate that they migrate independently; yet it seems prob- 

 able that in life there may be a slight amount of migration in 

 this way. Individual germ cells and cysts are usually not in 

 contact, so that it is a little difficult to conceive of these shifting 

 to a more cranial position as a result of the pressure of the cells 

 or cysts against one another as they increase in number. Since 

 the follicle cells show ameboid processes, it may be that the 

 germ cells are carried along by a movement of these. 



Some of the germ cells in the lamprey, as well as in other forms, 

 never reach the germ gland, but remain in the entoderm or some 

 other part of the body. Some of these cells may divide and form 

 cell nests in other organs. Such cell nests have been found in 

 the lamprey, both in the fat body and in the median and lateral 

 portions of the mesonephros (figs. 32, 33). The fate of these 

 cell nests is not known, but they probably degenerate. 



3. A third method of migration appears to have been observed 

 by Swift ('14) in the chick embryo. He says that the large 

 yolk-laden germ cells, which are first found in the germ-wall 

 entoderm, are taken up by the blood-vessels and carried by the 

 blood-stream to the germ-gland region. In fact, the germ cells 

 may be carried to any part of the body, but it is only in the 

 germ-gland region that they migrate out of the blood-vessels. 

 Swift ('16) has followed the later history of these cells and 

 has come to the conclusion that they give rise to the definitive 

 germ cells. Von Berenberg-Gossler ('14), who worked on the 

 early germ cells in duck embryos, also found cells similar to 

 those described by Swift, but he expresses a doubt as to whether 

 or not they are germ cells. He says: '^^lles in allem bin ich der 

 Ansicht, dass das ganze Verhalten dieser Zellen in hoherem 

 Grade davor warnt sie fiir Keimbahnzellen zu halten" (p. 261). 



