EXPLANATION OF PLATES 



All figures of plates 1 and 3 were made by the aid of a camera lucida. These 

 plates have been reduced in reproduction to three-quarters the original size; 

 the magnifications given for each figure are the actual magnifications as repro- 

 duced. The figures of plate 2 are copied from the sources indicated. 



PLATE 1 



EXPLANATION OF FIGURES 



1 to 4, Campanulariaflexuosa, approximatelj' X 1150; 5 to 8, Tubularia crocea, 

 X 620. 



1 Young planula with coelenteron present. Primitive ectoderm and ento- 

 derm present, but cells walls are to be detected in only a few places. Both 

 ectoderm and entoderm are filled with yolk spheres. 



2 Older planula with walls becoming more plainly marked. There are few 

 interstitial cells in the ectoderm, but a number are present in the entoderm. The 

 cell with the large nucleus may be like Wulfert's germ cell in Gonothyraea, 

 but the nucleus is similar to that of other entoderm cells. Some of the interstitial 

 cells are differentiating into gland cells, muscle cells, and the like. No germ 

 cells are present. 



3 Same planula as figure 2, showing only a portion of the entoderm. A group 

 of typical interstitial cells is represented. 



4 A still older planula with definitive ectoderm and entoderm. There are 

 fewer entodermal interstitial cells than in earlier stages. The interstitial cells 

 are undergoing differentiation, but no germ cells are present. 



^ Section of egg about the end of cleavage. The germ layers have not been 

 separated. The cell in division may be forming an ectodermal and an interstitial 

 cell. 



6 Embrj^o with a definite outer layer of cells and a solid central mass of cells. 

 Two interstitial cells have been produced, one of which was formed by the 

 division of a cell of the central mass. 



7 A later embryo with cubical ectodermal cells, and groups of intersitial 

 cells. One of the ectoderm cells is dividing to form an interstitial cell. None 

 of these interstitial cells form germ cells at this time. 



8 An embryo with coelenteron, about the period of the formation of tentacles 

 and the production of an actinula. The ectoderm and entoderm cells are dis- 

 tinctly separated by a supporting lamella. Groups of interstitial cells are 

 present and others are forming from the ectoderm. These interstitials form 

 nematocysts and other structures, but not germ cells. 



54 



