Germ-Cells in Chrysomelid Beetles. 275 



of both male and female pronuclei, after, or during their conjuga- 

 tion, while in Chrysomelid beetles the pole-disc is already fully 

 formed before fertilization. 



Another interesting variation in the origin of the germ-cells was 

 described by Boveri (1892) in Ascaris. After the first cleavage 

 division of the egg of this Nematode one cell preserves its chromo- 

 somes intact, while the other casts off the swollen ends of its chromo- 

 somes into the cytoplasm. During the first five cleavage divisions, 

 one cell retains the two full chromosomes, while all the others con- 

 tain the reduced amount. At the thirty-two cell stage, the cell con- 

 taining the two full chromosomes is given up entirely to the produc- 

 tion of the reproductive organs; the other cell gives rise only to 

 somatic tissue. 



The value of the pole-disc in the development of the insect egg 

 can only be surmised. Bitter (1890) maintains that the first 

 cleavage nucleus is hidden among the granules of the ''Keimwulst," 

 and he is inclined to believe that the first cleavage division separates 

 the primitive germ-cell substance from the somatic material, although 

 he could not demonstrate this. ISToack (1901) suggests that the yolk 

 elements of the "Dotterplatte" may hasten the growth at the posterior 

 pole of the egg, and that later they may possibly increase the vigor 

 of the pole-cells. That the pole-cells need special means of nourish- 

 ment is doubtless the case, for contrary to the condition in the 

 blastoderm-cells they are, at an early period, entirely separated from 

 the yolk, and later use up energy in their migration. 



Primary pole-cells are evidently characterized by the presence 

 of yolk material, as may be illustrated by the following citations. 

 In Chironomiis nigro-viridis Weismann (1863) found four oval 

 nuclei lying in the "Keimhautblastem" at the posterior end of the 

 egg', each of these, he says, '^besassen einen kreisrunden, klaren, 

 etwas rothlich schimmernden Kera, und in einigen Lagen ausserdem 

 noch ein oder zwei Dotterkornchen." These are the "Polzellen." 

 In another Dipteron, Simula sp., Metschnikoff (1866) records four 

 or five pole-cells which "bestehen ausser einem Kerne noch aus 

 einer die feinsten Dotterkornchen enthaltenden Zellsubstanz." The 

 same author (1866) also states that when the pseudovum in the 



