Germ-Cells in Clirysonielid Beetles. 285 



egi>'; there arc fifteen to twenty in Calliphora (JSToack, 1001), and 

 four to five in Simula (Metschnikoff, 1866) and in Pulex (Packard, 

 18Y2), althongli neither of the latter was examined carefully. 



Pole-cells are also found in several Chrysomelid beetles. Lecaillon 

 (1898) made no attempt to count the number of "cellules sexuelles" 

 in Glytra, but states that they are the cells which first reach the 

 "Keimhautblastem" at the posterior end of the Qgg. In Callujrapha 

 I have shown that the primitive pole-cell nuclei may be recognized 

 when they are four in number, but that these divide twice before 

 they separate from the Qgg, i. e., there are sixteen which pass through 

 the pole-disc. After separation these divide twic^', giving rise to sixty- 

 four. This number remains constant until the embryo is nearly 

 ready to hatch (Fig. 45) ; then the germ-cells increase rapidly by 

 mitosis. 



The very early stages of pole-cell formation were not observed 

 by me in Lepti)wtarsa. When the pole-cells were first seen in this 

 species, they formed a group lying at the posterior end of the egg 

 (Fig. 26) ; they are in every way similar in appearance to those 

 found in CaUigrapha (Fig. 24, Stage B), l)eing amoeboid in shape 

 and containing a layer of granules which they have gathered from 

 the pole-disc (Fig. 2). Embryos similar to Stages A to O were ex- 

 amined and in every case the germ-cells w-ere discovered occupying 

 a position which corresponds almost exactly to that found in Cal- 

 Ugrapha. Wheeler (1889), in his work on Leptinotarsa, not only 

 failed to find the pole-disc, but also overlooked the pole-cells at the 

 posterior end of the egg. His Figs. 66, 67 and YO represent surface 

 views of embryos like my Stages C, D and F, of CaUigrapha. The 

 group of pole-cells is present in every one of these stages in Lepino- 

 tarsa, and I cannot understand why Wheeler failed to find them. On 

 page 321 Wheeler (1889) says: "Sections taken in all directions 

 through the egg show the blastoderm to be of even thickness over the 

 whole surface (Fig. 63)." This is not true of the eggs of Leptinotarsa 

 I have examined, as there are two or three layers of cells at the 

 posterior end at the stage to which he refers. The space beneath 

 the pole-cells remains free from blastoderm-cells, and later becomes 

 the pole-cell canal just as we found in CaUigrapha, 



