INITIATION OF DEVELOPMENT IN EGG OF ARBACIA. 405 



to insemination. The egg nucleus may be located in either of the 

 " pseudo-blastomeres " ; or, as is frequently the case, it may he 

 seen lying in the constriction between the " pseudo-blastomeres." 



An interesting figure found in sectioned material deserves pass- 

 ing notice. In this case the spindle is in late anaphase ; one pole 

 with a chromosome group is in the larger of the " pseudo-blasto- 

 meres," which doubtless formed a membrane, since all the eggs in 

 this lot had membranes, and the other pole with a group of chromo- 

 somes is in a minute protuberance that could easily pass as a polar 

 body. A spermatozoon was found in the cortex at the pole oppo- 

 site the " polar body." This picture may be worthy of more than 

 passing comment. 



Observations on various ova (Myzostotna, CJicctoptcms, Dcn- 

 talinm, Ainphio.rus, Clcpsinc, etc.) show that the ectoplasmic layer 

 is absent at the outer end of the maturation spindle. Moreover, 

 Chambers by microdissection has shown that the region over the 

 outer end of the maturation spindle in the egg of Ccrcbratiilus is 

 very fluid. If we assume an absence of cortex in this region (or 

 perhaps physical or chemical difference), polar-body formation 

 would be comparable to this extrusion of endoplasm through the 

 cortex that I have mentioned above. 



Observations likewise show that in various ova (those of dona, 

 Cynthia, Chert opt cms, Ncmatodes, etc.) the cortex as well as the 

 endoplasm flows toward the vegetative pole. In Chatoptcrus, for 

 example, the ectoplasmic waves are clearly visible in the living 

 egg. It is not wholly impossible, therefore, that the definitive loca- 

 tion of the maturation spindle at the animal pole is owing as much 

 to the energy of these downward movements as to the movement 

 of the spindle itself. It is as if the egg substance flows away 

 from the animal pole, leaving the spindle behind. On this assump- 

 tion the size of the polar body would thus depend upon the size of 

 the more fluid ectoplasmic defect at the animal pole, the energy of 

 the downward movement of the ectoplasm, or both. Conklin's 

 production of large polar bodies in the egg of Crcpidnla through 

 centrifuging might be cited as evidence that an unusual bulk of 

 material may be thrust out of the egg as a polar body. 



Usually polar bodies do not fertilize. In ova in which matura- 

 tion normally takes place before fertilization this failure to fer- 



