656 JOHN BEARD, 



in the text- figure, they have probably recently arisen by mitoses of 

 one of the larger "raegaspheres" of about 0.05 mm. 



In the 7th row of sections the mesoblast is continuous with the 

 hypoblast, and the notochord is fused with the latter. There are now 

 only one or two of the cells we are dealing with: these all lie on 

 the top of the yolk, and there are none in the "mesoblast" of the 

 proliferating zone, or "primitive streak". The cells of this latter are 

 one and all smaller than primary germ-cells. 



Lastly, there are no germ-cells either large or small on the 

 blastoderm posterior to the embryonic foundation. Actually, therefore, 

 there are only one or two germ-cells in this embryonic foundation, 

 that is to say, in the proper position, and their wanderings have 

 hardly yet commenced. In all there are only two germ-cells 

 in the position of the future germinal nidus. 



In Raja hatis No. 690, where the medullary plate measured 3 mm, 

 and in which 14 mesoblastic somites seemed to be present, not a 

 single germ-cell was found within the embryonic foundation. The 

 blastoderm in front of the latter was not cut into sections, and, un- 

 fortunately, the sub-germinal structures and much of the lateral parts 

 of the blastoderm were removed. So that, as the result, apparently (!) 

 there is an entire absence of these cells. 



The moral, pointed by this fact and by the things, revealed by 

 the method of staining in No. 691, is plainly, that a thing is not 

 necessarily non-existent, because it has not been seen. It may have 

 been removed in the preparation, or the stain employed may have 

 been unsuitable for the revelation of its presence. 



The description of No. 691 has been somewhat detailed and, 

 therefore, long, but the facts laid bare fill in an important gap i) in 

 the observations. Nay, they completely bridge over the gulf between 

 blastoderms, in which there is no degree of embryo-formation, and 

 embryos of 5 to 7 mm, in which the medullary folds have recently 

 closed. 



XI. General Survey of the History of the Grerm-Cells. 



Eigenmann ('92) has concluded from the sizes of the germ-cells 

 of Cymatogaster = Micrometrus aggregatus, that they probably represent 

 cells of the 5th division of the egg. At the moment, although the 

 germ-cells of Raja can be definitely traced in preparations to the 



1) This coixld now (Dec. 1901) be bridged over many times. 



