MONOGRAPH OF THE EXISTING CEINOIDS. 483 



radiates out in several very fine pseudopodialike processes. In other cases the 

 plasma has become more extensive, though similar in form. The branched 

 processes of different cells may unite with each other and thus form a plasmatic 

 net traversing the sievelike calyx plates. 



Apparently not all the nuclei of the ectoderm cells remain in the living state. 

 In addition to such as show the typical form of the resting stage there are found 

 others which clearly are beginning to dissolve. In the deeper layers it can not be 

 determined with certainty whether these originally belonged to the outer layer or 

 to the mesenchyme. 



The degenerative changes are exhibited in different ways. Sometimes there 

 is a shriveling of the nucleus and a decrease in the amount of chromatin; the 

 nuclei appear small and pale with an indefinite contour, the chromatin granules 

 being either absent entirely or appearing as irregular isolated peripheral specks. 

 On the other hand, here and there strikingly large nuclei are to be seen, likewise 

 feebly stained and without chromatin granules or chromosomes, but containing 

 vacuoles or accessory iwlusions and always surrounded with a very small amount 

 of plasma. 



There is a possibility that these degenerative changes are not entirely normal, 

 since they occur only in individual instances and are not by any means general; 

 but at all events an increase in the number of nuclei through division at this stage 

 is most unusual. 



As during this period the surface area of the body is considerably increased 

 it necessarily follows that the cells become more and more separated, and at the 

 same time nuclear division becomes less and less frequent. 



In the living larvae individual cells of a yellow color stand out prominently 

 in the body wall and, less frequently, in the ectoderm of the tentacles. Except 

 for the color these resemble in every way the colorless cells, but on being treated 

 with reagents they seem to shrivel more readily, and they lose their spindlelike 

 shape more easily than the others. 



It is possible, though not proved, that these are identical with the yellow 

 cells of the free-swimming larvse. They lack the protruding club-shaped processes 

 so characteristic of the others. 



In the column also the histological relationships of the ectoderm are only 

 slightly different from those in the preceding stage. A cross section through the 

 portion of the stem adjacent to the calyx shows a number of spindle-shaped and 

 threadlike cells, lying more or less at right angles to the surface, with some star- 

 shaped cells distributed among them. There is no sharp demarcation between 

 ectoderm and mesenchyme. The whole external surface is covered with a trans- 

 parent glassy cuticle, which at the base may reach a considerable thickness. 



Vestibular lining and nervous system. At the end of the developmental period 

 just described there could be differentiated on the ectodermal vestibular wall three 

 sections (1) a posterior and outer, showing a pavement epithelium; (2) the 

 anterior basal wall, which goes over into the esophagus: and (3) the outer covering 

 of the tentacles, which develops from the border zone of the basal region. 



