Germ-Cells in Chrysomelid Beetles. 269 



the pole-eells, as soon as they have entered the embryo, begin to sepa- 

 rate into two groups ; three of them, in the figure, are present at 

 either side of the median line. A reconstruction made from several 

 series of transverse, sagittal and frontal sections, shows the pole- 

 cells distributed on either side of the last two segments of the tail- 

 fold (Stage L). 



The next embryo I shall describe (Stage M) has broadened 

 throughout its entire length ; it has also shortened, this being espe- 

 cially noticeable in the posterior portion. This contraction brings 

 the tail-fold nearer the posterior end of the egg than we found it 

 in the previous stage (Stage L). N'ow that the pole-cells have become 

 an integral part of the embryo I shall call them germ-cells. These 

 still lie in the last two abdominal segments, but have become partially 

 surrounded by mesodermal-cells (Fig. 49). They appear in sagittal 

 section (Fig. 41) to be closer to one another than we found them in 

 Stage L (Fig. 40). In transverse section (Fig. 49) they are shown 

 more clearly separated than before into two groups lying on either 

 side of the tail-fold. Part of them have reached the inner end of 

 the coelomic cavity, while the others are apparently moving in that 

 direction. Three kinds of cell (germ-, mesoderm- and ectoderm- 

 cells) from Fig. 41 are shown much enlarged in Fig. 57. The 

 germ-cells still exhibit all the characteristics that the pole-cells for- 

 merly possessed, and, in addition, contain a second nucleolus. The 

 cells of the mesoderm are smaller ; they appear darker than the 

 germ-cells due to a greater susceptibility of their cytoplasm to stains, 

 and the larger relative number of chromomeres in their nuclei. The 

 columnar ectoderm-cells contain cytoplasm which is even darker 

 than that of the mesodemvcells, and their oval nuclei possess a 

 smaller number of chromomeres, regularly distributed. The ease 

 with which these difi'erent cells can be distingiiished is evident from 

 a glance at the illustration (Fig. 57). 



A transverse section of an embryo slightly older than Stage ]\r 

 (Fig. 50) shows four germ-cells lying in a single row close to one 

 another; they all have now reached a point near the inner margin 

 of the coelom and have acquired an epithelial covering of mesoderm- 

 cells. A further shdrtening of the embryo brings the tail-fold close 



