GERM-CELL HISTORY IN THE BROOK LAMPREY 33 



the mesoderm in this region develops at the expense of these 

 cells. He says: 



Wenn aber das dorsale Mesoderm entstanden ist und bis zum 

 Teloblast reicht, tritt es in dieselbe einige Verbiiidiing damit wie der 

 Neuralstrang und die Chorda, und nachdem die Segmentierung des 

 Mesoderm bis zum Schwanzende fortgeschritten ist, erganzt sich der 

 jeweilig hinterste Abschnitt des Mesoderm durch Zellen die aus dem 

 Teloblast stammen. 



Hatta ('92, '07) describes certain cells as budding off between 

 the ectoderm and the entoderm in the region of the blastopore. 

 These cells he calls the 'peristomal mesoblast.' He could not 

 find any cells that corresponded to Kupffer's teloblast cells. It 

 is possible that Kupffer's teloblast cells and Hatta's peristomal 

 mesoblast cells are identical and that they correspond to the 

 large yolk-laden cells which later become included in the meso- 

 derm and form the germ cells. 



3. Discussio7i. a. Early segregation. We have seen that the 

 germ cells in Entosphenus wilderi may be traced to the large yolk- 

 bearing cells which at first are located in the mesentoderm. This 

 is in agreement with the observations of Wheeler. The history 

 of these cells, previous to their inclusion in the mesoderm is not 

 known. They apparently lie among similar yolk-bearing cells 

 belonging to the entoderm, and it is a question whether or not 

 they are essentially different from these. The germ-plasm 

 theory, as expressed by Weismann, demands a segregation of 

 the germ cells at a very early stage, or their origin, at least, 

 from cells that have never taken any part in the formation of 

 body tissues. In one sense all the yolk-bearing cells of the 

 entoderm may be considered as undifferentiated cells, but only 

 some of these cells which are included in the mesoderm become 

 germ cells. Most of the mesoderm cells, however soon begin 

 to divide, become smaller, lose their yolk, and form various 

 tissues, while the large cells that become germ cells do not 

 change in the least for a very long time. This indicates that they 

 are endowed with certain qualities which distinguish them from 

 the cells that become somatic. 



