516 BULLETIN OF THE 



for example, in mammals) the origin of the ovum is less directly trace- 

 able to an epithelial surface. Although there are not many existing 

 observations which are sufficiently connected to allow a definite conclu- 

 sion to be drawn from them in this particular,* there are evidences 

 which in many cases point strongly to the existence of the relationship 

 suggested. 



One of the results of a polar differentiation of the primitive axis is 

 often a difference in the specific gravity of the hemispheres, but it has 

 not been found that the conditions are the same in different animals. 

 Thus the primary pole is lighter in the eggs of frogs and the fowl, and it 

 has recently been shown to be the same in certain fresh-water pulmo- 

 nates (Rabl '75, p. 223), in Unio (Rabl '76), and in Clepsine (Whitman 

 78«, p. 29) ; while in a Gadoid studied by Haeckel ('75, p. 436) it was 

 found to be heavier than the secondary pole, thus causing the more 

 active hemisphere to be directed downward during the early stages of 

 development. In the study of fish eggs, probably nearly related to 

 those seen by Haeckel, Ed. van Beneden ('78, p. 43) observed a slightly 

 modified exemplification of the same thing. 



The activities of the primary pole are partly of a more constant nature 

 and widely disseminated, partly either less regular and .unlike in different 

 animals, or perhaps entirely absent. To the former belong the production 

 of polar globules, which will be considered later. Of the less constant 

 phenomena, perhaps that which has been most often described is the 

 pseudopodia-like elevation of portions of the cell protoplasm. This, 

 however, may be only a special manifestation of a more general amoeboid 

 activity, not always resulting in the formation of pseudopodia, but 

 usually causing the concentration of the active portions of the egg at 

 special epochs in its development. Such concentrations are especially 

 conspicuous in cases where there is an early and sharply expressed 

 separation of the formative from the nutritive constituents of the 

 yolk. This has been observed, for example, in the Ctenophorse just 

 before segmentation by A. Agassiz, Kowalevsky, and others; in osse- 



* A manifest obstacle is encountered in the difficulty of determining after its de- 

 tachment the position which the egg hekl during its growth. Unless the germina- 

 tive vesicle is prominently eccentric both before and after the liberation of the ovum, 

 or the deutoplasm is conspicuously segregated before maturity, or the envelopes of 

 the egg present some differentiation (as, for example, a micropyle) which can serve as 

 a constant point of reference, it may be impossible to reach a satisfactory conclusion. 

 Observations of an axial differentiation in liberated eggs are sufficiently numerous, 

 but the orientation of the differentiated axis in the ovarian egg has rarely attracted 

 the attention of observers. 



