6 GEORGE T. HARGITT 



ganstern find, what is probably more or less universal in hydroids, 

 that some of the interstitial cells remain undifferentiated for a 

 long time. But such undifferentiated cells are not germ cells, 

 since they form nettling cells throughout the life of the polyp 

 and probably act as replacing cells for any of the epithelial cells 

 destroyed. 



In order to test further this question of the presence of germ 

 cells in embryos, I have made a careful, extensive, and entirely 

 new study of the cleavage stages and planulae of Campanularia 

 flexuosa and Gonothyraea loveni ; also a similar study of cleavage, 

 embryo, and young polyp (actinula) of Tubularia crocea. In this 

 investigation I have followed the formation of the germ layers, 

 the differentiation of interstitial cells, and especially have searched 

 for primordial germ cells. 



In Campanularia and Gonothyraea cleavage results in the for- 

 mation of a solid morula composed of yolk-laden cells whose 

 boundaries are made out with great difficulty, if at all. The 

 outer cells of the morula arrange themselves into an indefinite 

 ectodermal layer, and later the cells of the solid central mass pull 

 apart to form an enteron, but during this time none of the cells 

 take on a columnar form and no interstitial cells are present. 

 Figure 1 shows the appearance of the embryo after the formation 

 of the enteric cavity; the cells are not sharply outlined, and 

 the nuclei, surrounded by masses of cytoplasm, are irregularly 

 scattered through both the outer and inner layers. This rather 

 indefinite condition is replaced in young planulae by the con- 

 dition shown in figure 2. The ectoderm cells are now columnar 

 and a few interstitial cells are present, the cells of the entoderm 

 are assuming a columnar form, and deep in this layer are groups 

 of interstitial cells. The boundary between the primitive germ 

 layers is not a definitely formed supporting lamella, but only the 

 cell outlines of the ectoderm. One is immediately struck by the 

 appearance of some of the interstitial cells of the entoderm, 

 and there is little doubt that some of these are similar to the 

 primordial germ cells of Wulfert. However, some of these are 

 spindle-shaped or stellate in form and their nuclei do not differ 

 from the nuclei of the epithelial cells of the entoderm. As the 



