Page:Proceedings of the Royal Society of London Vol 60.djvu/57

Rh ¥ i g. 6.—Section of stage of about the fourth day of fixation. cells are lacking. The gastral cells are a t first elongated, b u t later become shorter, and take on the characteristic appearance of collar cells. I have not been able to m ake out w hether all the g astral cells become collar cells, or w hether some of them do not become the wandering cells of th e adult, which seems very probable. The osculum appears about the sixth day of fixation.

These three species have larvae of th e type w ith w hich we are fam iliar from the descriptions of M etschnikoff * and S chm idt,f namely, oval ciliated blastulae, in which an inner mass is formed by immigration of cells into the interior. The process is most easily followed in the more transparent larva of L. reticulum (fig. 7), where the modification of ciliated cells into granular cells, and th eir subsequent immigration, takes place at the posterior pole. W hen the larva is ready for fixation, a considerable quantity of granular cells has been formed, though the cavity is far from being obliterated. In the opaque larvae of L. cerebrum and coriacea the process is more

XXIII. t “ Das Larvenstadium von Ascetta clathrus und Ascetta prim ordialisf ‘ Arch. f. Mikr. Anat.,’ vol. 14, pp. 249—263, Taf. XY, XYI.
 * “ Spongiologische Studien,” ‘ Zeitschr. f. Wiss. Zool.,’ vol. 32, p. 362, Taf.