MICHIGAN ACADEMY OF SCIENCE. 121 



groove now is overgrown Ijy the amnion, and the germ-band grows around 

 the posterior end of the egg and up on the dorsal surface carrying the germ- 

 cells with it. The germ-cells now begin to migrate from the amniotic cavity 

 into the embryo through an opening in the ectoderm which we may call 

 the germ-cell canal. They are amoeboid (Fig. 3) and their pseudopodia 

 are pointed in the direction of their migration. The pole-disc granules in 

 these germ-cells have been getting smaller during the migration and this 

 is the last stage in which they can be clearly recognized. The germ-cells 

 eventually all pass into the embryo. Here they separate into two groups, 

 one on either side of the germ-band; each group later l^ecomes a germ-gland. 

 The sex of the emliryo may be recognized liefore hatching by the shape of 

 the germ-gland. 



The history of the primordial germ-cells as outlined above gives, I think, 

 reasonable ground for the following conclusions. 



I. All the cleavage nuclei in the eggs of the above named beetles are 

 potentially alike until in their migration they reach the periphery. Then 

 those which chance to encounter the granules of the pole-disc are differen- 

 tiated by their environment, i. e. the granules, into germ-cells; all the other 

 cleavage products become somatic cells. The granules of the pole disc are 

 therefore either the germ-cell determinants, or the visible signs of the germ- 

 cell determinants. 



II. The primordial germ-cells pass entirely out of the egg and later 

 migrate into the eml^ryo by means of amoeboid movements. 



III. The sex of the embryo can be determined before hatching. 



Part II. The Result of Removing the Germ-Cell Determinants. 



A number of experiments were made in an effort to extract the germ-cell 

 determinants or primordial germ-cells from the egg. Eggs in the stage shown 

 in Fig. 1 were punctured with a needle in the center of the posterior end, 

 and a drop of cytoplasm containing the germ-cell determinants allowed to 

 flow out. Eggs thus operated upon produced embryoes and larvae which 

 were apparently normal but on examination were found to lack either all 

 of the germ-cells (Fig. 4) or to contain a fewer number than are jjroduced 

 normally. All of the germ-cell determinants probably were not removed 

 from those embryoes which still contained germ-cells. 



Other eggs were operated upon when they had reached the stage represented 

 in Fig. 2. The embryoes and larvae wdiich developed from these eggs either 

 lacked germ-cells entirely or possessed a fewer number than the normal. 

 The results of these experiments strengthen the conclusions reached by 

 morphological methods. 



Zoological Laboratory.. 



University of Michigan, April 1, 1909. 

 16 



