8 GEORGE T. HARGITT 



A similar solid morula is produced by the cleavage of the egg 

 of Tubularia crocea, but the cell outlines are sharply defined and 

 the formation of the germ layers is easily followed. During the 

 separation of an outer ectoderm from the superficial cells of the 

 morula, divisions take place (fig. 5) in such a fashion as to result 

 in the production of interstitial cells. But interstitial cells are 

 also formed by divisions from the deeper cells which make up 

 the entoderm (fig. 6). In this early stage the interstitial cells 

 may, but not always do, show a more deeply staining cytoplasm. 

 The ectodermal layer becomes more distinct, its cells become more 

 columnar, and the interstitials increase in numbers to produce 

 the appearance represented in figure 7. The groups of inter- 

 stitial cells are formed by divisions of the earlier cells, but others 

 are also formed from the cells of the outermost layers. By the time 

 a cavity is present in the center of the embryo the ectoderm has 

 become separated by a supporting layer (fig. 8). At this stage 

 the interstitial cells are numerous, so closely packed as to render 

 their outlines indistinct, and for the most part there is no dif- 

 ference in the staining reaction of the ectodermal and interstitial 

 cells. From this embryonic condition the young polyp or actinula 

 develops. In the development of the polyp the interstitial cells 

 shown in figure 8 are easily followed through their differentiation 

 into nematocysts and other specialized cells. At no time can 

 cells be found which resemble germ cells. This conclusion is in 

 harmony with the earlier results of the author ('09) on this 

 species, for it was then found that germ cells first became dif- 

 ferentiated in the medusoid buds of the hydroid from the ecto- 

 dermal cells. 



Other hydroids, as Clava, Hybocodon, Eudendrium, were 

 examined in cleavage and larval stages, but the material was not 

 sufficient in amount to permit a determination of the details 

 noted for Tubularia, Campanularia, and Gonothyraea. No 

 evidences of germ cells were seen in these stages of the forms 

 mentioned. A study of Clava and Eudendrium was made by 

 C. W. Hargitt ('04 b, '06), and the formation of germ layers and 

 interstitial cells was determined. In neither did a differentiation 

 of germ cells occur in the planulae or earlier stages. Other inves- 



