THE STRUCTURE OF XENIA HICKSONI. 295 



it is V-shaped. The filaments have a very sinuous course, and 

 are therefore cut across two or three times each in the same 

 section. These filaments are undoubtedly derived from the 

 lower end of the stomodseum, with which they are perfectly 

 continuous. Their cells are exactly like the cells of the stomo- 

 dseum, being small and having homogeneous or finely granular 

 deeply staining protoplasm and dark nuclei. Their cells differ 

 markedly from the cells of the surrounding endoderm, which 

 are larger, have reticulate protoplasm, stain more lightly, and 

 have less distinct nuclei. 



There is another very interesting feature worthy of note in 

 this polyp, viz. that sexual cells are already clearly diff'eren- 

 tiated. Sections through the mesenteries in the lower part 

 of the coelenteron show that a considerable number of the 

 endoderm cells covering the mesenteries are more spherical, 

 and two or three times as large as their neighbours. They 

 have clearer protoplasm than the ordinary endoderm cells, and 

 their nuclei are large (5 /x in diameter) and vesicular, and 

 possess a well-marked, deeply staining nucleolus (fig. 36 shows 

 them in a slightly older polyp). These cells occur in all the 

 mesenteries, but are more numerous in the ventral and lateral 

 than in the dorsal ones. Some of the cells have already 

 migrated into the mesogloea of the mesenteries, but most of 

 them still retain their position in the endoderm. These modi- 

 fied cells are present only in the portion of the mesenteries 

 situated in the lower two thirds of the coelenteron. 



On comparing these cells with the primitive sperm cells of 

 the adult the resemblance is so striking that I am convinced 

 these modified endoderm cells are the sexual cells which have 

 become differentiated at this early stage in the development of 

 the polyp. This view is supported by the following facts : 



a. They are at first endoderm cells covering the mesenteries, 

 which, on becoming diff'erentiated, migrate into the mesogloeal 

 lamina of the mesentery. This agrees with the origin, 

 migration, and position of the sexual cells traced in the adult 

 polyps (see p. 263). 



b. These cells are found in the mesenteries some distance 



