80 GERM-CELL CYCLE IN ANIMALS 



tive ability of these animals and must also account 

 for the development of plasmodia formed by dis- 

 sociated cells (Wilson, 1911; Muller, 1911) into 

 adult sponges with all specific characteristics in- 

 cluding reproductive bodies. 



It therefore seems possible that there may exist 

 in the sponges a continuity of the germ-plasm and 

 that the germ-cell material is distributed among 

 thousands of cells (tokocytes, see Table, p. 71) 

 which are derived from archseocytes, and that under 

 proper conditions these tokocytes may produce 

 oogonia or spermatogonia, or may aggregate to 

 form gemmules or regenerative bodies. This wide 

 distribution of the germ cells is what might be 

 expected in such lowly organized animals. Figure 

 28 shows the probable history of the germ cells in 

 the PoRiFERA from one generation to the next. 



2. CCELENTERATA 



The origin of the germ cells in the Ccelenterata 

 has been a much debated subject among zoologists 

 for three-quarters of a century. As early as 1843 van 

 Beneden undertook to determine the germ layer 

 from which the germ cells arise and concluded that 

 the ova originate in the entoderm and that the 

 spermatozoa come from the ectoderm. F. E. 

 Schulze (1871) claims that in Cordylophora both 

 the ova and spermatozoa are of ectodermal origin. 

 Kleinenberg (1872), working on Hydra, announced 

 that the germ cells are interstitial in origin and, 

 since the interstitial cells arise from the ectoderm, 



