GERM CELLS OF COELENTERATES 15 



become typically functional ectoderm and entoderm cells, then 

 later undergo a regressive change and become typical growing 

 cells which results in the formation of a new polyp after a later ' 

 second differentiation, there is nothing strange in thinking of 

 similar regressive changes of entoderm resulting in the formation 

 of germ cells. 



The earliest indication of cells clearly distinguishable as 

 sperm mother-cells was found in the entoderm of the stem of 

 the hydroid not far distant from the base of the pedicel of a de- 

 veloping gonophore. Figure 14 shows these cells in the ento- 

 derm, with rather deeply staining cytoplasm and large nuclei 

 which contained a good deal of chromatin concentrated close 

 along the nuclear membrane. A large nucleolus was present. 

 These sperm mother-cells arise in relatively small groups deep 

 in the entoderm, but whether by modification of entoderm cells 

 or by division of entoderm cells could not be determined. Such 

 cells divide rapidly to produce larger masses of spermatogonia 

 (fig. 15) which may be so numerous as to extend in unbroken 

 lines from the stem along the pedicel and into the developing 

 gonophore. As the growth of the gonophore continues, most of 

 these spermatogonia are found in the gonophore itself and fewer 

 in the pedicels and stems. Mitotic divisions are abundant in 

 all such groups of spermatogonia, whether in the stem, pedicel, or 

 developing gonophore, but most abundantly in the cells within 

 the gonophore. 



As the gonophore first forms, the spermatogonia are in con- 

 tinuous rows of groups, as shown in figure 17, but as growth 

 continues and the gonophore becomes more definitely formed, 

 these ceils are more distinctly massed (fig. 18), and there is an 

 ultimate localization into distinct testis-like groups (fig. 19). 

 The character of the spermatogonial cells of young developing 

 gonophores is shown in figure 16. Divisions are taking place, 

 some growth is evident, but all are of the stage of spermatogonia. 

 At about this period the cells undergo a synizesis-like change 

 and rapidly thereafter transform into spermatozoa. 



