STUDIES ON GERM CELLS 407 



closely allied to one of the species that I investigated (L. deceni- 

 lineata) in which a typical pole-disc like that shown in figure 7, 

 D occurs. Furthermore the cells which Wieman designates as 

 pole cells have none of the characteristics of the pole cells 

 described by other writers. 



Several important results have been obtained by experiments 

 that I have performed with the object of determining the char- 

 acter and significance of the pole-disc. When the freshly laid 

 eggs of Leptinotarsa decemlineata are centrifuged with the 

 posterior end toward the center of revolution the pole-disc is 

 moved gradually toward the outer anterior end as shown in 

 figure 8, G,H ,1, g.c.d. The movement en masse of the pole-disc 

 granules proves that they are heavier than the oil globules of 

 the vesicular zone (v.z.) and indicates that they do not form an 

 adventitious accumulation but constitute a definite structure 

 of sufficient rigidity to withstand the dispersing effects of a 

 strong pull exerted during a period of at least four hours. It 

 was hoped that by means of centrifugal force the pole-disc 

 could be located in a part of the egg different from that normally 

 occupied and that experimental proof of the necessity of their 

 presence for the formation of germ cells might thus be obtained, 

 but the abnormal development of the eggs prevented an accu- 

 rate determination of the various cells in sections of these centri- 

 fuged eggs. 



Two sets of experiments were undertaken in an attempt to 

 deprive the eggs of the pole-disc. If the embryo developing 

 from an egg without the pole disc failed to possess germ cells 

 the obvious conclusion would be that the pole-disc ' consists of 

 real germ cell determinants, necessary for the differentiation of 

 germ cells. In the first set of experiments the freshly laid eggs 

 were oriented and then pricked with a sharp needle in the center 

 of the posterior end. Since the egg is turgid a small drop of 

 the contents was forced out. Eggs treated in this manner con- 

 tinue to develop producing embryos and larvae apparently 

 normal. Sections of these seemed to show that less than the 

 characteristic number of germ cells were present. My inability, 

 however, to determine whether all of the pole-disc had been 



JOURNAL OF MORPHOLOGY, VOL.25, NO. 3 



