MUSEUM OF COMPARATIVE ZOOLOGY. 547 



this observation, and prefer to rest in the tentative belief that the thick- 

 enings are produced solely by molecular rearrangements of the nuclear 

 substance, which is accumulated along the protoplasmic axis of the 

 fibres. This view is not to be confounded with Auerbach's karyolysis^ 

 for it does not involve a dispersion of the substance of the nucleus 

 through the neighboring protoplasm, and its re-collection. It implies a 

 direct transfer of substance, but in elements too small to be individually 

 visible even with the best optical aids. 



Polar Globules. — The formation of polar globules has been shown 

 to be a constant feature in the maturation of eggs in representatives of 

 a majority of the recognized groups of animals. It is principally in 

 such as are characterized by the possession of a large proportion of nu- 

 tritive substance that their presence has not been established. The in- 

 creasing evidence of their constant production warrants the assumption 

 that they will be discovered in many cases, perhaps even in the larger 

 groups, where they have not yet been seen. The great probability of 

 their formation in Tunicata has been shown from the figures given by 

 Strasburger.* Still, the thoroughness with which some groups have been 



* P. S, — Grobben ('79, p. 209) has shown that in one of the Cladocera (Moina) 

 there is imbedded in the yolk at the animal pole of recently excluded eggs a body 

 which stains intensely in carmine. He believes it is a polar globule, which in this 

 case has not been detached from the yolk. This is perhaps the best evidence yet 

 produced to prove the existence of polar globules in Crustacea, and with further study 

 may possibly serve to show some of the regressive steps by which their production 

 has sunk from the dignity of cell division to a simple elimination of an amorphous 

 mass at the primary pole of the yolk. I would call attention, however, to the pos- 

 sibility of a close relationship between this body and the polar accumulation of stain- 

 able substance (polar ring) described by Whitman in Clepsine. The indistinctness 

 of its limitation from the yolk [loc. cit., Fig. 3), and its not protruding above the 

 surface of the same, are points of resemblance with the polar rings ; but its persist- 

 ence in late stages of segmentation (Figs. 5, 6) seems at first to indicate a different 

 fate from that of the ring substance. But when there are fifteen blastomeres, the 

 small sphere which contains the " Richtungskorper " is divided by an equatorial 

 plane into two spheres of equal size, and Grobben adds (p. 212): " Mit dieser Thei- 

 lung verschwindet der Richtungskorper von der Oberflache des Eies, da er offenhar 

 in die Tiefe der oberen FurchungsTcugeln gelangt. " 



This seems to afford strong indications of the identity suggested, and, if once estab- 

 lished, may remove Grobben's doubt as to the polar-globule nature of the bodies de- 

 scribed by Leydig ('60, p. 145) for Daphnia longispina as " einige blasse Kiigelchen, 

 ganz vom Charakter jener unter dem Namen * Richtungsblaschen ' beschriebenen 

 Gebilde," which he saw appear "an dem einen Pol ausserhalb der Eischale." It is 

 noticeable that at the first segmentation this body (Grobben, Fig. 2) remains in con- 



