The Germ-cells of Lophius. 569 



which affirm a mesoblastic origin at a later time. The sum total 

 of evidence at the present gives reasonable grounds to believe 

 that the germ-cells of vertebrates arise only as primitive cells set 

 apart during cleavage stages. 



2. Migration of Germ-cells 



a. Means of migration. As it has become apparent that the 

 germ-cells have their origin in parts of the embryo more or less 

 remote from the location of the future germ-glands, it has become 

 equally apparent that they undergo, a corresponding change of 

 position. There is quite general agreement that such change 

 of position is due in part to active migration of the cells themseh es 

 and in part to unequal growth and change of form of the embryo. 

 In some cases the germ-cells are seen during a part of their his- 

 tory to be distinctly amoeboid, an observation which suggests 

 how this migration is accomplished. By far the greater number 

 of writers, however, have spoken of them as of rounded form 

 through at least the greater part of their history. Allen ('06) 

 suggested that possibly even these rounded cells may be able to 

 execute migration by slight movements insufficient to give them 

 distinctly amoeboid form. 



h. Path of migration. The general course of migration is the 

 same in all species so far studied, though there are differences of 

 detail characteristic of each species. 



c. ^Lost'' and degenerate germ-cells. An interesting observa- 

 tion is that in many cases studied a considerable proportion of 

 the early germ-cells for some reason fail to complete the migra- 

 tion and find lodgment in some other part of the body. The fate 

 of these aberrant cells has not been followed, though there is 

 evidence that many of them degenerate. Beard ('02) found in 

 the skate that some of them segment to form little clusters of 

 cells which led him to beheve that very probably the further 

 development of an occasional one may produce teratomata or 

 dermoid cysts. 



