470 



application of my finds in the skate to Torpedo also, the possibility 

 of differences in detail must not be lost sight of. 



In his earlier writings on these cells Rückert was disposed to 

 assign to them a limited role in the formation of the blood. In his 

 most recent memoir he guardedly hints at a possible connection with 

 the merocytes; and thus he suggests a genetic relationship to super- 

 numerary spermatozoa ('99, p, 677). 



In citing Rückert's statements concerning them, one must care- 

 fully distinguish between his description of the megaspheres and his 

 surmises as to their nature and fate. The former, as might be 

 expected, is masterly. If one may judge from the brief reference ta 

 them in his recent memoir, RtJCKERT does not at present see his way 

 to any definite statements as to their nature and fate. That yolk- 

 laden cells, some of them of rather large size, are concerned in the 

 formation of the first blood-vessels and blood-corpuscles — the latter 

 being rounded (Minot) and not oval — is a fact not to be gainsaid. 



The writer has devoted some attention to this point, for during 

 the present investigation the larger work on the thymus and the first 

 leucocytes has not been forgotten. 



From Rückert's account of them it is very difficult to say what 

 a megasphere is, or to apply his description to the mass of cells 

 under the blastoderm of such an embryo as no. 434. Of Torpedo- 

 embryos of this very period Rückert defines the megaspheres as 

 follows: "Scattered between these elements there occurs a second form 

 of cell (megaspheres), characterised by their rounded form, their varied, 

 mostly very considerable size, their wealth of yolk-particles, and 

 very heterogeneous, often peculiarly arranged nuclear contents," He 

 notes their abundance in the immediate neighbourhood of the segment- 

 ation-cavity ; and further on, as in his latest work (where it is only 

 suggested as possible), he identifies them with merocytes, budded off 

 from the yolk ('87, p. 162). 



In this point it may be doubted, whether research will confirm 

 Rückert's surmise. It would, indeed, be remarkable, after what he 

 himself has recently established concerning the repulsion exercised 

 by cells of the blastoderm on the merocytic nuclei of the extra- 

 spermatozoa, were any of the merocytes to get into the embryo ('99, 

 p. 677 to 692 etc.) 



In some other particulars our results are in total agreement. 

 These concern the formation of nests of cells (concentric corpuscles) 

 in certain megaspheres, the frequent tendency of the nuclei to free 

 themselves of the burden of the yolk, and, at a date soon after Flem- 



