MUSEUM OF COMPARATIVE ZOOLOGY. 513 



the primitive axis are often so unlike, that the differences are con- 

 spicuous to the naked eye, and have therefore been long known, — as in 

 the eggs of many batrachians, where one pole is pigmented, the other 

 less or not at all. The relation of the poles to subsequent stages of de- 

 velopment has also, in some cases, been long recognized. Hence have 

 arisen for them the numerous designations animal and vegetative, active 

 and passive, foymq.tive and nutritive. 



There are objections to the employment of most of the names that 

 have been used. Thus, while active is an eminently fitting expression for 

 the pole where phenomena in such variety are exhibited, the opposite 

 pole cannot with as much propriety be said to be passive, since it is in 

 many cases the seat of peculiar, although thus far unexplained activi- 

 ties. Formative and nutritive are doubtless preferable to the older terms 

 animal and vegetative ; but to speak of that pole which embraces the 

 peripheral aster of the archiamphiaster as formative is to attribute to 

 its substance a function which is confessedly inapplicable to a considera- 

 ble portion of it, viz. to the substance of the polar cells, which univer- 

 sally take no part in the formation of the embryo. It seems to me, 

 therefore, that the substitution of less contradictory expressions is de- 

 sirable. I would suggest the use of primary and secondary,* as being 

 entirely consistent with our present knowledge. The active pole is cer- 

 tainly first in importance, and it probably is the first to be raised from an 

 indifferent condition. The nutritive pole is consequently of secondary 

 importance, as in point of time its phenomena succeed corresponding 

 conditions of the primary pole.t 



With the elevation produced by the first maturation spindle, at the 

 very latest, the /orm of the e^g evinces its monaxial condition, and the 

 primary pole is prominently specialized. The secondary, on the con- 

 trary, usually remains undifferentiated ; a few cases, however, have been 

 observed in which it is characterized by changes of form or structure. 

 Probably the most remarkable is that of Clepsine, so carefully studied 

 by Whitman ; but other instances of a peculiar modification of the 

 secondary pole have been seen, especially in the mollusks, and with in- 



* The corresponding radii may also be similarly designated primary radius and 

 secondary radius. 



t "Whitman ('78", p. 20), for example, says of Clepsine : " A short period of unipo- 

 lar activity is succeeded by a long period of bipolar activity which extends through the 

 cleavage stages. In the latter period the contrast between the two poles is still main- 

 tained; for the pole thus far active still asserts its pre-eminence by taking the lead in 

 actions that repeat themselves later and more sluggishly on the opposite pole. It is as 

 if one pole was trying to mimic the performances of the other." 



VOL. VI. — NO. 12. 33 



