480 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



A more developed columnal shows a sieve-formed plate with a circular central 

 opening, through which pass the extensions of the axial organ and of the cham- 

 bered organ, which in several places reach the outennost wall of the stem, where 

 they are covered only by a membrane instead of being overlaid with cells. The 

 longitudinal processes have become considerably longer and stronger, and at 

 different heights cross branches bind them together. With the growth of these 

 processes the plates become separated, and elongation of the column results. 



Although these longitudinal processes often overlap those of the neighboring 

 columnals, fusion never occurs between them. 



Sacculi. — From 24 to 48 hours before the opening of the calyx the sacculi first 

 make their appearance, one in each of the five angles of the calyx. 



Each sacculus originates from a small cluster of mesenchyme cells, which 

 forms on the outer side of the ring canal at the base of the middle tentacle of 

 each group. In cross sections through the larva they appear elongate, witli the 

 inner side lying closely upon a longer or shorter extent of the hydrocoele wall. 

 These clusters number from 8 to 12 cells, and they can be differentiated from the 

 neighboring mesenchyme tissue of the body wall only through the closer approxi- 

 mation of the cells. From the hydrocoele, vestibule, and oral coelome they are 

 sharply and clearly differentiated. 



When the required number of cells have assembled no further additions are 

 received from the mesenchyme, and the organ develops entirely from the circum- 

 scribed cell cluster. 



At this stage the rounded form of the constituent cells results in tlie presence 

 of a small gap between them, but there is no true lumen. 



Later the sacculus becomes somewhat enlarged, showing a definite wall 

 composed of a single layer of cells which are somewhat larger on the side toward 

 the hydrocoele than outwardly, a definite lumen, and a few cells lying within the 

 lumen and resting on the cells of the outer wall, some of which doubtless arose 

 through the intrusion of the cells of the outer wall into the lumen. But whether 

 some of these inner cells are derived from such of the cells in the original cluster 

 as from the first have occupied a more central position, or whether at one stage 

 all of the cells formed a continuous and even one-layered wall from which they 

 have passed inward, has not been definitely determined. 



At this stage all the cells are rich in plasma and show prominent nuclei, which 

 are somewhat larger than those in the mesenchyme cells. They stain more 

 strongly than the neighboring cells of the body wall. 



Before the rupture of the vestibule the inner cells have become considerably 

 more numerous. Nuclear division commonly occurs in the cells of the outer wall 

 in which the spindles are mostly approximately at right angles to the surface of 

 the saccidus, which appears to indicate that cell division will take place parallel 

 to this surface. 



In cross section the sacculus now appears as a strongly colored, well-circum- 

 scribed cell cluster, the elements of which, mostly rounded, are of different sizes 



