592 ARTHUR THOMSON. 
The Occurrence of Polar Cells.—Little has been done 
since Balfour wrote, in the way of extending our knowledge of 
the actual occurrence of polar cells in the different groups. 
Grobben has, however, observed what seems to be an undeni- 
able polar cell in Cetochilus, Billet has reported their 
occurrence in Rotifera, and Weismann (1) has Jately 
announced the existence of a distinct polar cell in the summer 
ova of some Daphniids. 
Other Extruded Elements.—On the other hand, nume- 
rous observers have noted the extrusion of protoplasmic 
elements from the ovum, which resemble in some respects the 
true polar cells, with which they have been repeatedly com- 
pared and confused. In the first rank among these, and as yet 
unique, are the pole cells of insects. They were first described by 
Robin in 1862, who compared them to true polar cells (vesicules 
directrices, Richtungsk6rperchen, directive bodies), and stated 
that they were incorporated in the blastoderm and utilised. 
Through the subsequent researches of Leuckart, Metschnikoff, 
Grimm, Weismann, and Balbiani, it has been shown that 
(following Balbiani’s (2) last report, 1885) two distinctly 
cellular elements, with nuclei apparently derived from the 
germinal vesicle, appear, usually successively, at the posterior 
pole of the ovum. In their further history they differ widely 
from polar cells, for they immediately divide into eight, and, 
after the blastoderm is differentiated, are carried in again by 
an invagination, and become, after fusing into four, the male 
or female reproductive organs. The re-entrance is denied by 
‘some but seems conclusively established: Weismann has also 
described distinct “ Richtungskorperchen,”’ but this is not cor- 
roborated by Balbiani, who notes the likelihood of mistake 
caused by the presence of ‘‘ protoplasmic drops ”’ at both poles, 
which originate according to him from the contraction of the 
vitellus. Weismann has, however, noted the presence of 
mobile amoeboid bodies in addition to the pole cells and 
alleged directive bodies. 
Non-cellular extrusigae—Apart from the special case 
of insects, extrusions somewhat comparable with polar cells, 
