568 BULLETIN OF THE 



explanations are not necessary. It appears to me that the one last stated is 

 the more probable, and that consequently there is no ground for doubting the 

 identity of the first amphiaster formed and that from the external half of which 

 the first polar globule arises. 



The eggs of Asterias are in some particulars less favorable for showing the 

 manner in which the polar globule is formed, than are those of Liraax. Nev- 

 ertheless the results correspond very closely. From the greater size of the 

 aster in Limax it has been possible to note more exactly the changes in the 

 course of its rays as the amphiaster approaches the surface of the vitellus. 

 Similar conditions doubtless prevail in Asterias ; for Fol says the external 

 aster (when the protuberance appears) is only a half-aster, since its centre 

 touches the summit of the protuberance. (Compare his PL II. Fig. 10.) 

 The internal half of the amphiaster, says Fol, alone remains in the yolk ; the 

 external half constitutes the protuberance, in which one still often sees rem- 

 nants of the bipolar rays. At other times the external half of the amphiaster 

 (spindle) promptly disappears, and is resolved into irregular corpuscles. These 

 remnants are, to judge from the figures, the fibre thickenings of the external 

 zone, which in the one case remain more regularly arranged ; in the other, less 

 so. The internal half of the aster, continues the author, preserves its structure 

 intact, and an elongated enlargement is seen upon each of the bipolar filaments. 



The detachment of this protuberance to form the polar globule, he thinks, 

 differs in several important pacticulars from the corresponding changes in seg- 

 mentation. Some of these differences are the same as those to which I have 

 already called attention in the case of Limax ; others I believe to be of less 

 general importance. " Nous ne voyons pas le globule s'arrondir au point de 

 lie toucher le vitellus que par une surface extremement petite, et s'affaisser, une 

 fois la division operee, comme c'est le cas dans le fractionnement ordinaire. 

 Le globule reste accole au vitellus par une surface relativement large et la 

 separation n'a lieu que tres-lentement, par un processus presque impossible a 

 observer directement." This appears to me principally dependent on the 

 presence of the " Ooleme pellucide," which envelops the ovum. It is rarely 

 that anything of a similar nature is observable in Limax (Fig. 57), where there 

 is nothing to interfere with the free course of the division. In the second 

 place, says Fol, the rays of the amphiaster (spindle) cut in two do not im- 

 mediately withdraw toward the centres of their respective asters to contribute 

 to the formation of the nuclei of two new cells. Those which belong to the 

 polar globule persist a long time distinct, and the varicosities remain some time 

 after the division is accomplished. Subsequently there are seen in the globule 

 granules and vacuoles, irregular both in form and arrangement. A long time 

 after the formation of the globule these parts are so arranged as to form a rela- 

 tively large nucleus surrounded with a layer of sarcode. It is unnecessary to 

 dwell on the significance of these peculiarities ; they are the same as in Li- 

 max. The re-formation of the fibre thickenings into a nucleus does not occur 

 with the same rapidity in all cases ; but in Limax it may occur almost as 

 promptly in the second polar globule (Fig. 60. rn.) as in the formation of the 



