238 liobert Wilhelm Hegner. 



four pol('-e(^]ls. He recorded tins interesting variation. "Manchnial 

 ahei- theilt sicli der Kern der ersten Polzelle nocli in der Schieht des 

 IJikUingsdottcrs liegend, so dass in dem Polraume auf einmal zwei 

 Polzellen erscheinen." The division of these pole cells into two 

 groups, each of which became surrounded by embryonic cells and con- 

 stituttd the germ-glands, confirmed Metschnikoff's (188G) account. 



According to Packard (1872) the eggs of Pulex canis, one or two 

 hours after deposition, contained "four distinct polar cells apparently 

 immersed in protoplasm, and a small indistinct one in addition." 

 They were distinctly nucleated and held in place by the vitelline 

 membrane. Thirty hours later the blastoderm was fully formed and 

 "Soon after this the polar cells break down and disappear." 



In another flea, Pulex fells, Balbiani (1875) found that the 

 "Anlage" of the reproductive organs was already visible after the 

 formation of the embryonic envelopes. It was a small naked mass 

 of clear cells lying in the posterior inner side of the abdomen. 



In a later work Weismann (1882) described events in the pole- 

 cell development of Chironoruiis sp. which diifered from those found 

 in C. nirjro-viridis in 1863. In Cliirouonws sp. the primitive pole- 

 cell nucleus was thought to have probably originated from elements 

 of the cleavage nucleus in the yolk nuiss and migrated to the surface 

 of the egg; there it entered a protoplasmic process of the "Keim- 

 hautblastem" together with a number of yolk-granules which flowed 

 in with it. This nucleus often divided, as Grimm (1870) had previ- 

 ously recorded, before the protrusion was entirely cut off from the 

 egg, resulting in two pole-cells. A second bud arose in the same 

 place as the first and passed through a similar process of separation 

 and division. Thus four cells were produced; these increased by 

 division to eight, and finally to twelve, which lay on the surface of 

 the posterior pole. They did not become indistinguishable from the 

 blastoderm-cells, as was described for Chiivnomus 7iigro-viridis, but 

 "spater wenn sich die Keimhaut gebildet hat, lagern sie sich deren 

 Oberfliiche loeker auf, oft zu zwei Gruppen formirt." After the 

 germ-band was formed two to four similar cells were found in many 

 eggs outside of the embryonic membrane, but their derivation from 

 pole-cells could not be determined. 



