49 



cells, termed by him megaspheres ; these he identifies as 

 mepocytes, budded off from the yolk. Beard proves them to be 

 in raja — germ-cells, or, if large, as the forerunners of such. 

 He writes: "Apart from the resemblance between these 

 " ' Megaspheres ' and germ-cells, there are also the facts of 

 " similar features in their degeneration. We cannot account 

 " for the small number of germ-cells in early embryos, or their 

 " absence from the mesentery, where later on, they are so 

 " abundant, without the inclusion of the wandering ' mega- 

 " spheres ' among the germ-cells. Moreover, there is such an 

 " unbroken transition from the ordinary germ-cells of my embryos 

 •' to the largest of the ' megaspheres,' and so many of the latter 

 " agree so absolutely ivith ordinary germ-cells in every respect 

 " except in position, that it is quite impossible to draw any line 

 " between them. In the normal position, in the germinal nidus, 

 " one also encounters abnormally large germ-cells or ' mega- 

 " spheres.' "* In the paper referred to, Beard at considerable 

 length establishes the identity. Accepting Beard's observations 

 as correct, it is certain that germ-cells precede the existence of the 

 embryo, and that they wander into the embryo later. 



Plate 1 : Illustrates the migration of the primary germ- 

 cells in a young embryo of Pristiurus, 4*5 m.m. in length. 



Plate 2 : Shows secondary germ-cells in normal and 

 abnormal situations in an embryo of Acaiithias, 21 m.m. in 

 length. Migration has ceased. 



Here it wmU be well to note that Beard has discovered an 

 important period in the development of the embryo which he 

 designates the "critical period." It is that time in the develop- 

 ment of the embryo when all the various organs are fully laid 

 down, but as yet are far from being complete.- -Of which, more 

 presently. — In the smooth skate this period is reached when the 

 embryo is about 70 m.m. in length ; the wanderings of the 

 germ-cells have ceased, and their division into secondary germ- 

 cells has commenced. In such embryos vagrant germ-cells are 

 rarely found ; they have probably for the most part degenerated. 



• John Beard, "The Germ-cells, Part \," Journal of Aiiiitomy and Physiology. 

 Jan., 1904; p. 22J. 



