504 



PEOP. ALLMAK ON THE STETJCTUEE AND 



hexagonal in transverse section, and closely adherent to one 

 another by their sides. These tubes form the foundation of the 

 chitinous float-ring ; they are, however, still open on their sum- 

 mits, and are each filled by the soft cell which acted as its matrix. 



The chitinous envelope of the disk has now considerably in- 

 creased in thickness, and along with the outer stratum of the 

 cystogenous layer has extended still further towards the middle of 

 the underside of the statoblast, while the inner stratum of the 

 cystogenous layer has entirely disappeared. 



The closure of the chitinous envelope, however, in the middle 

 of the underside of the disk is not yet completed, and there still 

 remains at this point a round hole through which part of the soft 

 contents of the disk protrude. As the hole grows gradually 

 smaller by the advancing chitin-deposit, the protruding mass be- 

 comes constricted off, and at last the disk becomes completely 

 enclosed in its chitinous envelope. 



Nearly up to the point of the complete closure of the chitinous 

 envelope the chitin had been laid down by new cells which are 

 being constantly formed on the advancing margin of the cystoge- 

 nous layer. The final disappearance of the central aperture, how- 

 ever, is effected by the cells, which already lie round its circum- 

 ference, bending over the aperture, cupola-like, and depositing the 

 chitin from their bases. 



The stage is now attained in the development of the statoblast 

 when the disk is completely enclosed in its chitinous envelope 

 and the chambers of the float-ring are laid down. These cham- 

 bers, however, are still open on their summits, and are filled with 

 their formative cells. The whole of these contents now withdraw 

 towards the peripheric or open ends of the chambers, so that 

 these are left empty, and the cells which had filled them now hang, 

 in the form of small nucleated lumps of protoplasm, on the epithe- 

 lial layer of the statoblast. 



These little protoplasmic masses become gradually broader, 

 press close to one another, and form with the remaining cells of 

 the cystogenous layer a continuous stratum all round the stato- 

 blast. This stratum now begins to secrete chitin from its entire 

 inner surface ; and by the chitinous deposit thus uniformly laid 

 down over the surface of the statoblast, the chambers of the float- 

 ring are closed above and the chitinous envelope of the disk 

 thickened. The fine tubercular sculpture which characterizes the 



