THE GERM-CELL CYCLE 275 



diminution processes resembling those occurring in the blasto- 

 meres of Ascaris. Efforts made by the writer to determine 

 the origin of the polar-plasm have thus far been fruitless. The 

 primordial germ cell of Miastor passes through three divisions. 

 The eight cells thus produced separate to form two rows of 

 four each ; these remain undivided for a long interval during 

 embryonic development, but finally undergo a definite number 

 of mitoses, namely three, resulting in two germ glands con- 

 taining thirty-two oogonia each. There are thus in this genus 

 a definite number of oogonial divisions (6), and it is no longer 

 necessary for us to express our ignorance by stating that there 

 are n divisions of the oogonia in the germ-cell cycle, since 

 here n = 6. It is obvious that the original polar-plasm must 

 be divided among the sixty-four daughter cells of the primordial 

 germ-cell, each of which receives approximately one sixty- 

 fourth of the original mass. During the growth period of 

 these germ cells the quantity of the polar-plasm contained in 

 the original mass is regained, i.e. the amount in the mature 

 ovum equals sixty-four times that contained in the oogonium 

 from which it developed. What causes this increase in amount, 

 and where does the new substance come from ? I (8) have sug- 

 gested (1) that the polar-plasm may increase by autodivision 

 of its particles during the growth period, or (2) that by its 

 presence it may cause the production of new substances like 

 itself; this activity ceases when an equilibrium point has been 

 reached, as is known to occur in the case of enzymes, thus 

 accounting for the rather definite quantity present in each 

 generation. 



An early segregation of the germ cells has been described 

 in many other dipterous insects ; of these perhaps the best 

 account is that of Hasper(9) on Chironomus. Near the posterior 

 end of the freshly laid egg of this midge is a layer of deeply 

 staining granules, the " keimbahnplasma." One of the first 

 four cleavage nuclei encounters these granules, becomes sur- 

 rounded by them as with a halo, and is cut off from the rest 

 of the egg ; thus is formed the primordial germ cell. No 

 chromatin diminution process was observed here such as 

 occurs in Miastor. During the division of the primordial germ 

 cell the granules of the keimbahnplasm appear to be equally 

 distributed between the daughter cells. The origin of the 

 granules was not determined. 



