MUSEUM OF COMPARATIVE ZOOLOGY. 549 



the vitellus. In the case of mammals there is sufficient evidence of the 

 existence of polar globules, as the early studies of BischofF indicate, but 

 no one seems to have yet discovered the method of their production, 

 and it is therefore open to question whether they arise by a process of 

 cell division from the external halves of maturation spindles. Ed. van 

 Beneden's account for the rabbit does not remove the uncertainty. 



There are, besides, two other groups of animals in which the presence 

 of anything even remotely comparable to polar globules has not yet 

 been satisfactorily determined, — Rotifera and Arthropoda. Flem- 

 ming's expectations in regard to the existence of the polar globule in 

 Lacinularia were not confirmed by BUtschli, who directed particular 

 attention toward their discovery in the Rotifera. The accounts of their 

 formation in certain Crustacea * also need further confirmation. 



The polar globules may be considered from three standpoints, — the 

 morphological, the physiological, and the historic, or phylogenetic. 



Morphologically viewed, there can no longer be any doubt that they 

 are cells. They are formed by a process in all essentials like ordinary 

 cell division. They are composed of a protoplasmic substance which 

 stains feebly, and of a nuclear substance which stains deeply. The lat- 

 ter is derived, through the intervention of a fibrous spindle and a divid- 

 ing nuclear plate, from the nuclear substance of the immature ovum. 

 The lateral zone of thickenings in the globule is not always massed into 

 a single nuclear structure. It is possible that in many cases this con- 

 dition is exhibited because sufficient time has not elapsed for the accom- 

 plishment of the successive acts of the consolidation. But in any event 

 the conclusion seems inevitable that there is a decline in the functional 

 activities of this cell, which delays the completion of its work beyond 

 the normal period of such changes. It is probable that many polar 

 cells never would have attained this typical condition, — a cell with a 

 single nuclear structure, — even if their activities had not been inter- 

 cepted by the action of reagents. A decline in the functional potency 

 of the polar cell is ultimately followed by a complete surrender of its 

 morphological integrity. That, however, does not warrant a denial of 

 its morphological value as a cell, any more than the gradual obliteration 

 of the structure of an element from the epidermis would justify a de- 

 nial of its cell character. 



Notwithstanding the cell nature of the polar globule, there is one mor- 

 phological peculiarity connected with its production, besides its diminu- 

 tive size, which has been previously observed, it is true, but which has 



* Consult Leydig '60, p. 145 ; Dieck '74 ; and Hoek '76, p. 62. 



