The Development of Theridium. 325 



cells lie between the entoblast and the mesoblast. With the disappear- 

 ance of the extraembryonic area formation of blood cells seems to 

 end, or at least I found no new centers of proliferation, probably 

 therefore new blood cells are from now on produced by division of 

 the old ; perhaps the small cells within the heart have been formed in 

 this way. 



10. The Germ Cells. 



I have given much time in the attempt to trace the origin of the 

 germ cells, but have reached only inconclusive results. In my paper 

 on the fertilization (1907) I described an extranuclear mass near 

 one of the nuclei of the four-cell stage, and suggested that such a body 

 might represent either abnormally placed chromosomes or else a 

 normal chromatin exclusion. '"In no other cells of the two-cell, four- 

 cell or eight-cell stage, either in the anaphase or the rest condition, 

 were bodies like these found, so that it is fair to conclude that the 

 two eggs first mentioned were abnormal." ISTow, I find extruded 

 chromatin masses in most of the cells invaginating at the anterior 

 cumulus, and some of these cells appear to originate within the cyto- 

 i^lasm structures somewhat similar to nuclei, a phenomenon that I 

 propose to treat specially at another time; but similar bodies occur 

 in what appear to be germ cells, so that their presence is not a sign of 

 somatic differentiation. 



No evidence of germ cell segregation could be found until the 

 later portion of the gastrulation process, and then in the region of the 

 blastopore of the anterior cumulus. In Fig. 28, PI. II, is exhibited 

 a cell with a nucleus much larger than those of any other cells 

 (G. C. ?) ; it borders on the gastrocoel, and may be the first definitive 

 germ cell, but the only reason for so supposing is the great size of its 

 nucleus. In the same situation is found a little later a gi'oup of 

 eight cells immediately lining the gastrocoel {G. C. ?, Figs. 37b, 38b, 

 PI. Ill) ; these are unbranched, thus differing from the early vitello- 

 cytes, they possess relatively clear nuclei and are much larger than 

 the early mesentoblastic cells (Mes. E., Fig. 37a). With the oblitera- 

 tion of the gastrocoel and the flattening of the anterior cumulus these 

 cells become indistinguishable, so that they either become branched 

 like vitellocytes or else by division become as small as the mesento- 



