256 C. O. WHITMAN. 



occurs. The relative distance of the araphiaster from the upper 

 pole of the egg is about tlie same in Chn^i'^iue as in the frog, and 

 the cleavage progresses from the up])er (oral) to the lower pole 

 alike in both cases. In the case of Hydra, where this one-sided 

 cl<^avage is carried to the extreme, it is highly probable that the 

 amphiaster lies much nearer the surface of the egg than in 

 Clepsine or the frog, and the radial iuiluence of the nucleus is 

 accordingly much more strongly manifested, producing the 

 pscudo])odes. The nearer the amphiaster lies to the middle of 

 the egg, the less marked will be these peripheral manifestations. 

 If it lie exactly in the centre, then the cleavage will appear in a 

 perfectly regular manner, the meridian depression encircling the 

 egg from the outset and progressing steadily from its periphery 

 to its centre. That no pseudopodes, folds, or wrinkles make 

 their appearance in such cases of unifornri cleavage, finds an easy 

 exjjlanation in the statements just made. 



d. Polar Globules (Robin). — rieinmiug (3S) , Fol (il), and 

 Biitschli (27) have given mure or less complete bibliographical 

 references on this topic, which renders it unnecessary for me to 

 go beyond the line of my remarks in this direction. 



The evidence in favour of the general occurrence of polar 

 globules in the animal kingdom is rapidly accumulating. The 

 earlier observations of 0. Ilertwig and Fol seemed to raise a 

 doubt in regard to Echinus and Asterias, but the latest investi- 

 gations of these authors, as well as those of van lieneden and 

 Giard, prove that these globules are of general occurrence among 

 Echinoderms.^ 



Although, according to the statements of different authors, 

 the number of polar globules varies from one to four or five, yet 

 it is evident that tivo is by far the more general, and perhaps the 

 normal number. This is the case in some Coelenterata, in 

 llirudinea (and many other worms), Echinodermata, a large 

 number of Mollusca, Petromyzon, and Mammalia. (). lli^twig 

 (^^-t) 1^'^s shown that two directive cells are normally produced 

 in Nephelis — whilst the third, which is sometimes seen, arises by 

 a division of the first. Later these globules were seen to unite 

 into a single discoidal body. This division and coalescence have 

 been observed in other cases, and they furnish an explanation of 

 the variation in number. 



1 I must here call attention to an oversiglit of O. TT('rtwi<». He lias 

 stated in at least two places (J, J,, -^y,,) that otlier authors have I'iiiled to liml 

 polar globules in tlie egg of Toxopucustcs. Ilertwig seems to have entirely 

 overlooked a classic memoir on tlu^ embryology of Echinoderms l)y A. 

 Acassiz (]), publisliod as long ago as JSOt. Agassiz has not only figured 

 tiieso globules, l)ut (lisliiictly slates (p. 7) that he lias found them in l)oth 

 Asteracanlliion and Tuxupiieuslcs. 



