CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 7G3 



of the cell (Figs. 41, 60) is usually flat, but may he concave (Fig. 58) 

 or convex (Fig. 42). The desniocytes arise, or at least attain their 

 differentiation, in the secreting epithelial layer. Cells in contact with 

 the axis (Figs. 41, 58, dfun'ci/.), that apparently are at first not diU'er- 

 ent from the secreting cells, broaden their axis-end, pushing other 

 cells away from the axis. To such a cell a mesogloeal process, prob- 

 ably secreted largely by adjacent epithelial cells, becomes applied, 

 so that the cell then appears to ])e simply a pi-olongation of the meso- 

 gloea. The nucleus of the cell persists for a long time, but often it 

 degenerates. Meantime the differentiation of the broad end of the 

 cell shows it to be a desmocyte. Secretion on the part of the sur- 

 rounding cells may continue around these desmocytes. Figin-e 42 

 shows that in this case much of the secreted layer of the axis was 

 formed after the differentiation of the desmocyte and while it was still 

 functioning as a hold fast. Figure 52 {(hm'cy'.), compared with 

 Figure 42, shows evidence that the axial portion of the desmocyte 

 may lose its connection w^ith the mesogloea owing to the constriction 

 of its neck by the formation of the horny secretion. It is in this way 

 that some of the smaller chambers of the axis-cortex are formed. 

 When this has taken place, other desmocytes appear in the same 

 region peripheral to it. 



In places where a great many desmocytes have been formed (Plate 

 4, Figs. 58, 60), the secreting cells are pushed back from the secreting 

 surface in disarray. The displacement is perhaps a necessary result 

 of the broadening of the ends of the desmocytes. At a later time, per- 

 haps in response to the same stimulus that causes the beginning of a 

 new skeletal chamber in places where desmocytes do not occur, such 

 displaced secreting cells rearrange themselves preparatory- to the 

 secretion of a new lamella, leaving a lenticular space between them- 

 selves and the previously secreted portions of the axis. Later still, 

 some sort of stimulus may then cause other desmocytes to appear 

 among these secreting cells, probably as the result of the differentia- 

 tion of a part of their own number. I consider these holding cells 

 to be homologous to those seen by Fowder in a madreporarian coral 

 and to those w^hose origin was described by Bourne ('99) for the mad- 

 reporarians and for Heliopora, an alcyonarian Avith a calcareous 

 skeleton; but I find no reference to similar cells for any other alcyo- 

 narian, except that possibly A. Schneider (:05, p. 128) found them; 

 but if so, he evidently thought them artifacts. I have found them in 

 all the colonies of Pseudoplexaura studied, and I have also seen them 

 in the species of Euniceopsis and Gorgonia which are associated with 



