2 7 6 SCIENCE PROGRESS 



The origin of the germ cells in certain chrysomelid beetles 

 has been shown by the writer (10) to be similar in its main 

 aspects. In several species of the genera Calligrapha and 

 Lcptinotarsa the posterior end of the egg differs from the 

 anterior pole in the presence of a disc-shaped mass of granules 

 which I first called the pole-disc, but later designated as germ- 

 cell determinants, or keimbahn-determinants. The sixteen 

 primordial germ cells (pole cells) originate at the time of 

 blastoderm formation at the posterior end of the egg, and each 

 becomes provided with a portion of the pole-disc. 



These germ cells at first form a group outside of the embryo, 

 but later migrate into the embryo and separate into two 

 divisions, each of which becomes the basis of the germ glands 

 on one side of the bod}'. Experiments (i i) have shown that if 

 the posterior end of a freshly laid egg is killed with a hot 

 needle, thus destroying the pole-disc and the substance in which 

 it is suspended an embryo develops lacking sex cells. A 

 similar operation after the primordial germ cells have been 

 formed likewise results in embryos without sex cells. This is 

 the earliest stage I believe in the development of any animal at 

 which surgical castration has been performed. 



Two groups of Crustacea are known to contain species 

 which exhibit an early segregation of germ cells. Haecker's(i2) 

 investigations on Cyclops have recently been repeated by Amma 

 (13"; and extended to include species from the genera Diaptomns, 

 Canthocamptus, and Heterocope. In these animals the stem cell, 

 which at the thirty-two-cell stage becomes the single primor- 

 dial germ cell, can be recognised by the presence within it of 

 a mass of granules, the ectosomes, which appear at one end 

 of the spindle during the mitotic division of the fertilised egg 

 and at a similar stage in the four divisions of the stem cell. 

 When the primordial germ cell divides, the granules are dis- 

 tributed apparently equally between the two daughter cells 

 (oogonia or spermatogonia according to whether the egg would 

 have developed into a male or female individual). In Diaptomus 

 ccernleus the ectosomes appear in the egg before the pronuclei 

 break down to form the first cleavage spindle. 



Kuhn's(i4) researches on the summer egg of the Cladoceron, 

 Polyphemus pediculus, show that in this crustacean the stem cell 

 can be identified by the remains of one or several nurse cells. 

 These nurse cells become embedded in the undivided egg, and 



