690 THE GENERATIVE ORGANS. 
blastoderm as the original polar cell. These four cells, which 
are at first outside the blastoderm at the posterior pole of the 
egg, pass in some unexplained manner into the yelk ; here they 
are seen lying, two on either side, embedded in a mass of cells 
above the primitive band. 
Metschnikow and Leuckart further believed that they traced 
the development of the ovaries to these cells. Their views 
were favourably received and strongly supported by Balbiani. 
In 1882 the latter indicated, in a brief communication to the 
Academy of Sciences in Paris, that he had traced the develop- 
ment of the gonads in the sexual forms of Chironomus to the 
polar cells, and subsequently published his observations in 
extenso [847]. 
Balbiani believed that he had actually traced the passage of 
the polar cells through the blastoderm, but his observations 
appear to me to be very doubtful on this point. He says: ‘At 
a certain moment the group of polar cells appear to be elevated 
above the posterior pole of the blastoderm and to penetrate 
the vitellus. First the posterior pole of the blastoderm ap- 
pears slightly flattened, then it surrounds the polar globules as 
if to enclose them in a depression, afterwards this depression 
is elevated into the vitellus, and its summit is seen to be sur- 
mounted by the polar globules and to be surrounded by the 
yelk. The mass has therefore passed from the exterior into 
the interior of the blastoderm.’ So far Balbiani seems to have 
observed a real phenomenon; it is only his subsequent re- 
marks to which I take exception. He asks how this passage 
is effected, and admits that the actual observation of the manner 
in which it occurs is rendered very difficult by the superposition 
of the polar globules and cells of the blastoderm in many layers 
and by the opacity of the yelk; like all observations made on 
organs and structures en masse, it is impossible to be sure of the 
exact disposition of the cells. Balbiani suggests two solutions 
—one is that the blastoderm (epiblast) is deficient in the region 
of the polar globules, and the other, which he apparently 
adopts, is the actual passage of the polar cells between the 
epiblastic elements. Balbiani’s observations are in favour, I 
