No. 3. ] DEVELOPMENT OF MARINE SPONGES. 299 
the non-spicular pole, and this area will gradually disappear, 
the disappearance taking place from the spicular pole forwards. 
Though I have not actually witnessed the transformation of 
columnar cells into flat cells, this is undoubtedly what takes 
place. Close observation fails to reveal the casting off of any 
portion of the larval ectoderm, and sections give every indica- 
tion that the columnar ectoderm is gradually transformed into 
a covering of flat unciliated cells. The replacement of columnar 
cells by flat ones never fails to take place in the manner de- 
scribed, z.¢., gradually from the spicular pole forwards. Now 
the surface area of one of the flat cells is considerably greater 
than that of a columnar cell and since the entire area to be 
covered remains approximately the same, it is obvious that all 
the columnar cells cannot be transformed into flat cells. What 
becomes of those that are not so transformed? A partial 
answer to this question is suggested by the very characteristic 
appearance of the anterior pole in the older swimming larvae 
(Fig. 30). As may be seen in this figure, the nuclei of the 
ectoderm cells are arranged in a dense zone, except at the 
anterior end, where they are much less densely packed, and 
where they form a columella-like mass projecting some distance 
into the interior of the larva. The cells composing this mass 
are so small that I cannot speak of their outlines with certainty, 
but they appear to be spindle-shaped. The mesoderm cells at 
this end of the larva are nearly all spindle-shaped, as may be 
seen in the figure, and the general appearance of the region 
suggests that the ectoderm cells are migrating at this pole into 
the interior of the larva. With my small store of facts this 
must remain a mere conjecture, and yet the point seems worth 
mentioning. 
After the larva reaches the stage shown in Fig. 27, it sinks 
to the bottom and attaches in the following manner. Keeping 
its spicular pole applied to the bottom of the dish and its long 
axis more or less vertical, it begins to rotate. The rotation 
lasts for several hours, and may be interrupted by the larva 
moving to a new quarter of the dish, there to begin again its 
monotonous rotation. All this time the transformation of the 
ectoderm is taking place. After the ciliated ectoderm has 
