126 



SCIENCE 



[N. S. Vol. XXXVI. No. 917 



primitive reproductive organs lying in their 

 definitive positions. From 1866 until 1908 

 nothing was added to our knowledge of the 

 embryonic development of these pffidogenetic 

 larvse. Kahle," however, has given a remark- 

 ably clear and detailed account of the subject 

 and I have already been able to confirm many 

 of his results. 



The entire " Keimbahn," as described by 

 Kahle, is shown in the accompanying dia- 

 gram. The primary oocyte contains, at the 

 posterior pole, a mass of protoplasm which 

 stains more deeply in aniline and carmine 

 than does the rest of the material in the egg. 

 This substance is named " polares Plasma." 

 Erom twenty to twenty-four chromosomes are 

 present. One polar body is given o£P, and this 

 divides once. One of the eight nuclei result- 

 ing from the first three divisions of the egg 

 nucleus becomes embedded in the " polares 

 Plasma" and is cut off by cell walls forming 

 the primordial germ cell. During the division 

 from the 4-eell to the 8-cell stage, three of the 

 nuclei (I., II., III.) lose part of their chroma- 

 tin, which is cast o£E into the cytoplasm, and 

 is called by Kahle " Chromatinreste." The 

 nest, or 15-eell stage, includes a single pri- 

 mordial germ cell which contains the " polares 

 Plasma " and possesses a large nucleus with 

 the full amount of chromatin, and fourteen 

 somatic cells, each of which lacks " polares 

 Plasma," and has a small nucleus from which 

 part of the chromatin has been cast out, and 

 which possesses only half the number of 

 chromosomes (10-11). 



The primordial germ cell undergoes six 

 successive divisions, thus producing sixty-four 

 oogonia of the 6th order. At the end of the 

 third division two germ glands are formed of 

 four cells each. The multiplication period is 

 followed by the growth period during which 

 each oogonium enlarges into a primary oocyte 

 with a nucleus containing from 21-24 chromo- 

 somes and with a mass of " polares Plasma " 

 at the posterior pole. The origin of the 

 " polares Plasma " was not discovered. 



MiastoT americana Felt agrees so far as 



• Kahle, W., ' ' Die Paedogenesis der Cecido- 

 myiden," Zoologica, Heft 55, pp. ]-80, 1908. 



I have been able to determine with the species 

 studied by Kahle (Miaster metraloas). The 

 "polares Plasma" is present; the primordial 

 germ cell is set aside at the 8-cell stage; and 

 two germ glands of four oogonia each are 

 formed. I have not, however, been able to 

 count the chromosomes accurately nor to find 

 division figures of early stages which show the 

 diminution of the chromatin. These stages I 

 hope to find later. 



The determination of the germ cells of 

 Miastor seems to combine two of the methods 

 that have already been described for other ani- 

 mals. The presence of a deeply staining mass 

 of material at the posterior pole of the egg, 

 which becomes a part of the primordial germ 

 cell but is excluded from the somatic cells, is 

 similar to the condition in Chrysoioelid 

 beetles, in Cyclops, and in Sagittaf whereas 

 the diminution in the amount of chromatin 

 which takes place in the somatic cells but does 

 not occur in the primordial germ cell is like 

 the casting out of the chromatin from the 

 nuclei of the somatic cells of Ascaris.' 



The " Keimbahn " of Miastor furnishes a 

 remarkably clear example of the continuity of 

 the germ plasm. It likewise enables us, at 

 least in one instance, to state the number of 

 cell divisions that occur during the period of 

 multiplication of the oogonia, and, indeed, the 

 number of cell divisions from one oocyte to 

 the sixty-four oocytes of the next generation, ■ 

 namely, ten. The writer is now engaged in an 

 attempt to determine the origin of the pecul- 

 iar substances (germ cell determinants) such 

 as the " polares Plasma " of Miastor which 

 have been observed in the primordial germ 

 cells of many species of animals, and hopes to 

 discover the role they play in the primary dif- 

 ferentiation of germ cells and somatic cells. 



Robert W. Hegner 



■Qniversitt of Michigan 



' Boveri, Th., ' ' Die Entstehung des Gegensatzea 

 zwischen den Geschlechtszellen und den somatischen 

 Zellen bei Ascaris megalooephala," Setz. Ges. f. 

 Morph. Physiol, Bd. 8, 1892. 



' Hegner, K. W., ' ' Germ Cell Determinants and 

 their Significance," Am. Nat., Vol. 45, pp. 385- 

 397, 1911. 



