THE STRUCTURE OF XEKIA HICKSONI. 225 



polyp, viz., that sexual cells are already clearly differentiated. Sections 

 through the mesenteries in the lower part of the ccelenteron 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 p. 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 mesoglcea of the 

 mesenteries, but most of them still retain their position in the endo- 

 derm. These modified cells are present only in the poi'tion of the 

 mesenteries situated in the lower two-thirds of the ccelenteron. 



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 differentiated, 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. 197). 



b. These cells are found in the mesenteries some distance below the 

 lower end of the stomodseum. They are not present in the mesenteries 

 of the free portion of the polyp, but in the portion enclosed in the stem. 

 They are therefore in the position in which the genital products of the 

 adult polyps are most numerous (see p. 207, and Fig. 8). 



(c) They agree in size and are similar (especially their large vesicular 

 nuclei) in appearance to the primitive genital cells of the adult. 



Fig. 36 shows a section through a ventral mesentary of a slightly 

 older polyp (No. V. in table, p. 215). The genital cells are still more 

 clearly marked than in the preceding polyp, and, as shown in the 

 figure, are readily distinguished by their large vesicular nuclei. Many 

 of them have already taken up their position in the mesoglcea. 



On comparing the polyp "8 mm. long with one about two-thirds its 

 size (*56 mm. long) it is seen that- during the period in which the 

 elongation of *24 mm. was accomplished, many important adult 

 structures were acquired, viz., the first indication of pinnules on the 



