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BULLETIN OF THE 



Stages of development in cases when no polar globules are formed. He 

 then urges in support of his " protective " theory, — (1.) that, as a rule 

 the polar globules accompany only the " inequal " method of segmenta- 

 tion ; (2.) that the place of their origin is always the animal pole of 

 the germ ; and (3.) that the specific gravity of the animal pole is less 

 than that of the opposite pole in cases of inequal segmentation. From 

 all this Rabl concludes that the polar globules serve the purpose of 

 elastic balls in preventing the dangerous pressure of the germ against 

 the membrane of the egg. For the ascidians with their primordial 

 segmentation, the pressure being uniform on all sides, not a few, but a 

 large number, of these elastic balls (Testazellen) are provided. 



However ingenious this theory may at first sight appear, it cannot 

 claim to be based upon satisfactory grounds, and I am the more sur- 

 prised that Rabl should have promulgated it in connection with his 

 previous paper on pulmonate mollusks, since in that case such a rela- 

 tively enormous distance intervenes between the " Eiweisshiille " and 

 the embryo, — a distance so great that one rarely finds the yolk even 

 in the vicinity of the membrane of the albumen until rotation begins, 

 and then, as the author himself admits, this protective function must 

 cease to exist. Apart from the absence of proof that such protection is 

 needed, or is even advantageous to the embryo, or that the polar 

 globules are capable of offering such protection, the links in his chain 

 of argumentation seem to be exceedingly fragile. All authentic obser- 

 vations, it is true, go to show that there exists the constant relation 

 between polar globule and the promorphology of the egg which Rabl 

 has expressed by saying the globule is formed at the animal pole of the 

 germ. That, however, is only the connecting link between two others. 



I believe the evidence is still wanting to prove that the animal pole of 

 the egg is specifically the lighter in all cases of inequal segmentation, or, 

 at least, that the difference in specific gravity is sufficient to cause the 

 germ to rest with the animal pole uppermost. My own observations 

 in the case of Limax have not afforded the least ground for such a 

 conclusion. The yolks of eggs left undisturbed for hours have been 

 found to present the same want of uniformity in position, which is met 

 with under any other circumstances ; individual eggs have been ob- 

 served for a long time during the early stages of segmentation, the polar 

 globules remaining all the time in such a position as to be seen outside 

 the profile of the yolk. Furthermore, it seems to me, this theory 

 necessitates the assumption that the yolk (or embryo) is specifically 

 lighter than the enveloping medium, otherwise there would be no 



