MUSEUM OF COMPARATIVE ZOOLOGY. 555 



appear very probable. If the polar globules exist simply to remove an 

 essential substance in order that another essential element may take its 

 place, the whole process would appear to be a waste of energies with 

 which nature is not often chargeable, unless it can be shown that some 

 serviceable end is reached by such an exchange.* 



None of the physiological interpretations offers any explanation of the 

 most characteristic feature of the polar globule, — its cell nature. Evi- 

 dently, any the6ry to be entirely satisfactory must explain the signifi- 

 cance of this fact. Even if it be granted that its present function is 

 one that may be accomplished without its assuming the condition of a 

 cell, it will be useless to attempt to elucidate its full meaning without 

 recognizing the importance of that peculiarity. The constancy of this 

 morphological characteristic points to one of two things ; either there is 

 some peculiarity in the present function of the globule, which is best 

 subserved by a cell-like structure, or it is simply the heritage from a 

 former state in which the polar cell may have had a different functional 

 signification from that to which it now responds. 



* P. S. — Balfour ('80, p. 63) suggests ^^that the function of forming polar cells 

 has been acquired by the ovum for the express purpose of preveniing i^cirthenogenesis.'" 

 His reasons for this conclusion are stated as follows : "The explanation given by 

 Mr. Darwin of the evil effects of self-fertilization, viz. the want of sufficient differen- 

 tiation in the sexual elements, would apply with far greater force to cases of par- 

 thenogenesis. " 



" In the production of fresh individuals, two circumstances are obviously favorable 

 to the species: (1.) That the maximum number possible of fresh individuals should 

 be produced ; (2.) That the individuals should be as vigorous as possible. Sexual 

 differentiation (even in hermaphrodites) is clearly very inimical to the production of 

 the maximum number of individuals. There can be little doubt that the ovum is 

 potentially capable of developing by itself into a fresh individual, and therefore, un- 

 less the absence of sexual differentiation was very injurious to the vigor of the 

 progeny, parthenogenesis would most certainly be a very constant occurrence ; and, 

 on the analogy of the arrangements in plants to prevent self-fertilization, we might 

 expect to find some contrivance both in animals and in plants to prevent the ovum 

 developing by itself without fertilization. If my view about the polar cells is cor- 

 rect, the formation of these bodies functions as such a contrivance. " 



"Why the eliminated substance takes the form of a cell, still remains as difficult of 

 explanation as before. But the principal obstacle to the acceptance of this hypothe- 

 sis is that the presence of polar globules in the fertilized eggs of Arthropoda and Ro- 

 tifera has not been satisfactorily established in a single instance, (compare, however, 

 the statements made above concerning polar globules in Crustacea, ) much less shown 

 to be an event of common occurrence. There can be no urgent reason for claiming 

 that there is an omission or suppression of an event not yet shown to have an 

 existence. 



