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, i.e., 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 



