GERM CELLS OF COELENTERATES 621 



The next older stage to be considered is one in which the bud 

 has increased in size and has the general shape of a tiny medusa. 

 The various parts are all present as rudiments except that the 

 jelly has not yet begun to form, but none of the organs or parts 

 is more than rudimentary. A section through the stomach 

 wall of such a bud is shown in figure 50. The entoderm cells 

 are as characteristically gastrovascular cells as are those in a 

 similar position in an adult medusa, but none of the other tissues 

 of the medusae are characteristic. The ectoderm of the stom- 

 ach (right in figure 50) is composed of a single layer except at 

 one point where a mass of cells extends part way around the 

 stomach. This group of cells was near the oral end of the 

 stomach but not at the tip; these cells may not be simply the 

 embryonic cells of the growing medusa, though there is practi- 

 cally no difference between them and such embryonic cells. 

 More than any other cells so far found and described, these cells 

 resemble primordial germ cells, though I fancy this resemblance 

 is assisted by our knowledge that this is the place where the 

 gonad is eventually to form. If we take it for granted that 

 these are primordial germ cells let us note at what a relatively 

 late stage they develop. The developing bud is of considerable 

 size, all the organs and parts of the medusa have been laid down 

 as rudiments and some parts, as the entoderm of stomach, have 

 taken on their definitive form and are functional. The gonad 

 does not even exist as a rudiment till relatively late in the 

 development after all these other parts are started. Further- 

 more the gonad is developed, not from any localized group of 

 cells but from the general mass of more or less embryonic tissue 

 which has earlier been the source from which all other tissues 

 and organs have taken their start. 



Since it has been shown that there are no recognizable germ' 

 cells in the tentacle bulb from wh£ch the medusa buds arise it 

 must be that the buds have come from the ordinary cells of this 

 region. The cells of the tissues of this budding region (figs. 

 40, 41) show beyond question that the cells here are differen- 

 tiated. By the very side of the developing bud the ectoderm 

 may be forming nematocysts (at the right in figure 45) and the 



JOURNAL OP MORPHOLOGY, VOL. 28, NO. 2 



