624 JoflN BEAkb 



be difficult to demonstrate a single germ -cell in an abnormal 

 situation. 



None the less, it is absolutely certain, that in such abnormally 

 placed germ-cells are present. Of the embryologists, who have studied 

 early Elasmobranch embryos, barely half a dozen have mentioned the 

 occasional presence of the big yolk-laden cells, which Rückert terms 

 "megaspheres". 



But for a certain period of the development, i. e., until most of 

 the mesodermal somites are laid down, it may be stated with the 

 utmost confidence, that few Elasmobranch embryos and blastoderms 

 of certain species are entirely free from them. 



Born thinks, and rightly, that our methods are at fault. By 

 many researches, beginning with those of Nussbaum ('80), and especi- 

 ally including those made on fishes, it has been established, that one 

 of the most striking features of the germ-cells is their retention of 

 yolk, long after it has disappeared from cells of the embryo. 



Our ordinary methods tend to conceal this peculiarity. The yolk, 

 wherever present, is almost hidden away in preparations stained with 

 carmine, or even with haematoxylin as usually employed for embryos. 

 Most embryos are treated by some modification or other of these 

 methods; and, therefore, in most embryos the germ-cells, in spite 

 of their size, are, if not hidden, veiled off from too close an in- 

 spection. 



In the present instance, although it had long been intended to 

 attack the germ-cells and their problems at some time or other, the 

 immediate cause of the investigation lay in the application of a method 

 of staining for a totally different purpose. Another reason acted as 

 an incentive. In the course of a prolonged research on the thymus 

 the working hypothesis of a morphological connection of germ-cells 

 and leucocytes suggested itself ^). It, therefore, became of importance 

 to enquire into the history of the germ-cells. Out of these two facts 

 the present writing has, almost at a tangent from other work, taken 

 its origin. 



Methods of Investigation. 



In these days, when the treatment of embryos for research has 

 attained the lank of a fine art, it might appear superfluous to record 

 how the material employed was prepared and treated. But the ex- 



1) It is, perhaps, needless to say, that there is no connection. 



