MUSEUM OF COMPARATIVE ZOOLOGY. 327 



liquefaction of these masses ; they are therefore composed of a mixture 



— in very different proportions in different cases — of the substance of 

 the old nucleus and the protoplasm of the cell. 



The observations made on Limax confirm much that is described by 

 Fol. I am more inclined to think the asters result from a fusion of some 

 part of the nuclear contents with the vitelline protoplasm, and that it is 

 the chemical activity thus brought about which induces the radial ap- 

 pearance, than to assent to so crude an explanation as is implied by an 

 irruption of the protoplasm into the nucleus, however much the figure in 

 Fol's paper on the Pteropods (PI. VIII. Fig. 4) may resemble such an 

 invasion. 



While I do not believe the whole difference between intra- and extra- 

 nuclear filaments can be ascribed to the media in which they are found, 



— which, according to Fol himself, become optically the same ! (see above, 

 p. 325), — the occurrence of thickenings in the extranuclear filaments is 

 sufficient to suggest a closer relationship between them and the spindle 

 fibres than has been generally admitted. My own studies are not ade- 

 quate, I regret, to either confirm or refute the observation respecting the 

 migration of the extranuclear varicosities. 



The part of Fol's description which agrees least with my own observa- 

 tions is that which concerns the relation of the new nuclei to the central 

 masses (areas) of protoplasm. 



The second paper by 0. Hertwig ('77) on " Bildung, Befruchtung und 

 Theilung," etc. embraces studies on the eggs of two of the Hirudinea (Hso- 

 niopis and Nephelis), and of Rana temporaria and R. esculenta. Although 

 this paper does not undertake the discussion of cell division and the ac- 

 companying changes of the nucleus, it indirectly has much to do wdth 

 these phenomena, since the conclusion is here for the first time reached 

 and formally presented, that the production of the polar globules takes place 

 in the manner of cell division. 



In a foot-note Hertwig (pp. 48, 49) says a few words about cell division 

 in frogs' eggs. Soon after the division of the segmentation nucleus, there 

 is found imbedded in the finely granular substance of each of the heads 

 of the dumb-bell figure a group of numerous large and small vesicles, 

 which are mutually flattened by reason of their closeness. These are 

 tinged in carmine, and possess the properties of small vacuolar nuclei. 

 They are produced from the separate granules of the condensation zone, 

 which simultaneously imbibe nuclear fluid. Each group therefore cor- 

 responds to a single daughter nucleus. 



The division of the yolk appears to be effected by the contraction of 



