30 WHITMAN. [ VoL. II. 
de la téte ne se trouve-t-elle pas concentrée en un point deéter- 
miné du corps ovulaire ?” 
Both views recognize the necessity of assuming that the 
course of ontogenetic development is in some way predeter- 
mined in the egg; but while one finds the force motrice in the 
nucleus, the other would locate it in the cytoplasm. The ad- 
vocates of the former appeal to the so-called isotropy of the 
cytoplasm, to the conspicuous part played by nuclear bodies in 
fecundation and cleavage, to the incapacity for regeneration 
shown by enucleate fragments of infusoria, etc. ; while the sup- 
porters of the latter insist on the constancy of premorphologi- 
cal relations (axial relations, relation of first cleavage-plane to 
the median plane of the future embryo), the remarkable struc- 
tural features exhibited in some eggs, cleavage in planes not 
previously marked by karyokinetic division, etc. The truth 
appears to me to lie on both sides, the error consisting only in 
unduly exaggerating the relative importance of one or the other 
factor. Just now the weight of authority seems to be turning 
in favor of the first view, a result which must be attributed very 
largely to the influence of recent discoveries and theories re- 
specting the nature of fecundation. The question is one of 
such fundamental importance, that it seems desirable to analyze 
closely the facts bearing on the subject. It is for this reason 
that I have dwelt more at length on the movements of the 
germinal vesicle and the pronuclei. 
Especially important is the study of the structure of the egg, 
and the modifications which it undergoes during the period of 
maturation. One of the most important contributions in this 
direction is unquestionably Van Beneden’s great work on Asca- 
ris. No other biologist has yet gone so deeply and thoroughly 
into the subject, nor has any one discussed it with a keener 
appreciation of its theoretical importance. Such well-marked 
structural features as are claimed to exist in this egg are incon- 
sistent with the idea that the cytoplasm is isotropic. 
Polar Rings. — Among the more extraordinary examples of 
cytokinesis may be mentioned the polar rings in the egg of Clep- 
sine, first described by Grube” (pp. 15-16), and recently more in 
detail by Robin # (pp. 97-105) and Whitman ® (pp. 20-29, 39-41). 
19 Untersuchungen ueber die Entwicklung der Clepsinen. Kénigsberg, 1844. 
1 Mémoire sur le Développement embryogénique des Hirudinées. Paris, 1875. 
12 The Embryology of Clepsine. Quart. Your. Mic. Sci., July, 1878. 
