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 

 cxtenso [347], 



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 



