752 PROCEEDINGS OF THE AMERICAN ACADEMY. 



being sharply outlined in regions where there are muscles; but only 

 occasionally, and then in the thicker parts of the layer, are there any 

 included cells. A thin layer of it is also found between the endoderm 

 of the mesenteries and the egg or the sphere of sperm mother cells. 

 In the body wall, particularly at the base of the polyp (Fig. 19), it is 

 thicker and here a few included cells appear. These cells (c/. ms'gl.) 

 are smaller than the ectodermal cells and have a correspondingly 

 small nucleus; they have several elongated, more or less branched 

 processes. Cells in the coenosarc (Fig. 28, cl. 7ns'gl.) which I inter- 

 pret to be the same as these are very numerous and show plate-like 

 expansions of the terminal branches, which have been described by 

 Kassianow (:08, p. 525) for Alcyonium. The mesogloea layer in the 

 coenosarc is very thick (Fig. 22); here the newer mesogloea, that 

 which is near the ectoderm, is less dense than the portion which occu- 

 pies the deeper layers. Sections stained in either eosin or Mallory's 

 phospho-tungstic haematoxylin show well the differences between 

 these regions. The ectoderm cells of the coenosarc are very long and 

 the interstitial cells at their bases very numerous. The latter have 

 the appearance of being pushed away from the ectoderm as growth 

 proceeds, and they arrange themselves, or are arranged, in masses or 

 cords (Figs. 22, 28, cl.cd.), in which the indiA'idual cells are often 

 only loosely associated. The interstitial cells (Fig. 24, cl. in.) w^hich 

 are still near the ectoderm, are globular or irregularly branched, but 

 otherwise they are not different in appearance from the ectoderm cells 

 of the coenosarc or of the polyp wall and tentacle ; but they, together 

 with the deeper and more specialized cells constituting the cords 

 (Fig. 22), reach down even to the axis epithelium. A transverse 

 section of such a cord is given in Figure 29; other sections are shown 

 in Figures 27 and 28. In these cords some of the cells {cl. in.) are like 

 the interstitial cells near the ectoderm. Others {cl. grn.') contain few 

 or many granules, which vary in size and staining capacity, but always 

 stain, either slightly or hea^•ily, in haematoxylin or in eosin. The 

 granules vary in size in different cells. In other respects these cells 

 resemble interstitial cells. Some of the cells at the edges of the cord 

 are partially surrounded by the jelly of the coenosarc, and these are 

 appreciably smaller and more elongated than the others. The loosely 

 arranged cells of the cord show no extra-cellular matter except at the 

 edges of the cord. Where such matter is evident, the characteristic 

 finely granular mesogloea cells {cl. ms'gl.) are to be found with 

 their greatly elongated processes and terminal branchings. 



I believe the cells enumerated below constitute a genetic series: 



