EMBRYOLOGY OF CLEPSINE. 285 



insects, at a time when the embryo is almost fully developed, is 

 full of cellular elements.^' In regard to the fate of these elements, 

 Dohrn states, in opposition to Kowalevsky, that they enter 

 into the composition of the embryo as cells {middle intesfine, 

 hlood, (^r.) 



The occurrence of free nuclei in the egg of fishes is established 

 by a large amount of concurrent testimony. Kupffer (oVf, Gas- 

 terosteus, Spinachia), Bambecke (8, Cyprinoids), Gotte (yot)> 

 Oellaclier (l%^), His (fi), and Klein (82, Salmo; Balfour 

 (a-o-ijL'xa-g^ Mustelus) and Schultz (4-yil4yT^ Torpedo), among 

 others, have reported such nuclei. 



There is a difference of opinion in regard to the origin of these 

 elements. Klein maintains that they are formed denovo. Balfour 

 leaves the question open as to whether they spring from pre- 

 existing nuclei, or have an independent origin. Oellacher and 

 Scliultz derive them from the cells of the blastodisc. In regard 

 to the role they play Balfour, Schultz, and Klein believe they 

 enter into the entoderm and mesoderm. The following remark 

 concerning the significance of these phenomena is from Balfour 

 (tVt) '• " ^^ ^i"6, therefore, forced to believe that the fine 

 granular, and probably the coarsely granular, yolk of this mero- 

 blastic egg (Mustelus), consists of an active organised basis with 

 passive //olk-spheres embeded in itP 



For the occurrence of such nuclei in the egg of birds, we have 

 the testimony of Gotte (- iV-tt), Rauber ( W , '/y-* ,7rT7ri.f -7 u-)> ^i'^ 

 Balfour (i^). Gotte saw what he interpreted as a process of 

 segmentation about these nuclei. His (tVt) ^^^^ Disse ('/v ) 

 think, on the other hand, that Gotte was misled by artificial 

 productions. Whether these formative cells enter into the 

 entoderm is not yet determined. 



From the references here given it will be seen, not only that 

 free nuclei are of very general occurrence, but also that they play 

 no unimportant part in the formation of the embryo in bilateral 

 animals. It is also plain, as Balfour and Lankester have stated, 

 that the yolk is interfused with active protoplasm and conse- 

 quently more or less subject to cleavage in one or another form. 

 The production of free nuclei is only an abridged form of 

 cleavage, and these nuclei undoubtedly have the same genetic 

 relation with the egg-nucleus as have the nuclei of the blastoderm. 

 That they form entoderm-cells in the case of Euaxes is quite 

 certain, and in Clepsine the evidence all points in the same 

 direction. This conclusion is, however, in plain contradiction 

 with the opinion of Grube, Rathke, Robin, and Hoffmann, all of 

 whom, however, overlooked the formation of entoplasts, and con- 

 sequently were, as I believe, misled. Grube (tt)> Rathke (ti5y)j 

 and lloffman Q^^) all derive the entoderm by delamination from 

 the blastoderm. According to Robin (-^-yj-l^-y^) the entoderm 



