32 PETER OKKELBERG 



SO that it appeared as though they might belong to it. He says, 

 however, "Eine genaue Untersuchung hat mich aber iiberzeugt 

 dass es urspriinghche jMesoderm-elemente und nicht etwa vom 

 Darmblatt her eingewanderte Zellen sind" (p. 53). 



Wheeler ('99) has given an excellent account of the early devel- 

 opment of the germ cells in the lamprey (Petromyzon planeri). 

 He recognized the germ cells in the posterior region of embryos 

 as early as my figure 5, the stage in which they were first observed 

 by me in Entosphenus wilderi. Wheeler found: ''Just laterad 

 to the myotomes a few very large rounded masses of yolk." He 

 described each mass as containing a nucleus and ''more or less 

 distinctly marked off from the adjacent entoderm elements." 

 He says further, "These large masses are the primitive repro- 

 ductive or sex cells. They can hardly be assigned to the meso- 

 derm because their appearance and position are those of entoderm 

 cells in this stage. Still they lie in a portion of the entoderm 

 which becomes mesoderm with the more lateral extension of the 

 latter layer." 



Beard ('02), in an attempt to work out a numerical law for 

 the primordial germ cells in animals, says that the number of 

 cells should in each case be 2'^1. His theory is that the blasto- 

 derm in animals corresponds to the sporophyte in plants, and to 

 it he applies the term 'phorozoon.' After a time one of its 

 cells divides a definite number of times and forms the primordial 

 germ cells. The number of divisions varies according to the 

 species. One of these primordial germ cells is sacrificed to 

 form the embryo so that the actual number of germ cells remain- 

 ing is in each case 2"-l. In the case of the lamprey (Petromy- 

 zon planeri) Beard finds that 2"" = 32 and that therefore in this 

 species the number of primordial germ cells is thirty-one. 



Interesting in connection with the description of the early 

 history of the germ cells in the lamprey is an observation made 

 by Kupffer ('90). In the early gastrula of Petromyzon planeri, 

 he found, between the ectoderm and the entoderm in the region 

 of the blastopore, certain cells which he called 'teloblast cells.' 

 They were easily distinguished from the yolk cells adjoining 

 them, but their origin was not observed. Kupffer thinks that 



