36 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



The flattening of one side in the embryo represented in figure 1, plate xv, 

 may indicate the formation of the vestibulary invagination, but it can not 

 be ascertained beyond doubt ; the pressure of the embryos against one another 

 within the marsupium may have caused it, as it has doubtless caused the 

 similar flattening in the much younger stage represented in plate xiv, figure 5, 

 in which one could hardly think of seeing the beginning of the vestibulum. 



3. THE DIFFERENTIATION OF THE LARVA. 



During this period important structural changes take place, leading to 

 the organization of the fully formed larva. Figures 4 to 1 1 of plate xv serve 

 to illustrate this period. 



In its outer appearance the embryo becomes more elongate. The ves- 

 tibulary invagination appears as a depression on the ventral side (plate xv, 

 figure 5) and likewise a depression is formed in the anterior end of the embryo 

 representing the suctorial disk (plate xv, figure 11). A distinct cihation is 

 seen in this depression. The ciliated bands are developing, the nuclei being 

 arranged in more or less distinct groups corresponding to the bands (plate 

 XV, figures 9 to 11). Glandular cells have developed in the ectoderm, espe- 

 cially at the anterior end (plate xv, figure 10), but they may be numerous 

 also in other places (plate xv, figures 5, 7, 11). The nervous system has just 

 begun to differentiate (plate xv, figure 10). 



The entoderm shows a conspicuous difference from the preceding stage, 

 the lumen being very small or even completely obliterated. The ceU limits 

 have mostly disappeared, and especially it is important to notice that the 

 cells are not distinctly limited against the lumen of the entoderm, as they 

 are in the previous stage (plate xv, figures 6 to 11). A sort of dissolution 

 accordingly takes place, the yolk spherules wandering into the lumen, where 

 they are evidently being absorbed. The nuclei, which in the preceding stage 

 were lying fairly regularly at the inner border of the entoderm, are now 

 spread over the whole entoderm mass, without order, evidently after having 

 undergone division. This does not correspond to the inwandering of the 

 phagocytes, as is, evidently, the meaning of K. A. Andersson, who points 

 out (op. cit., p. 6) that this process occurs here at a much earlier stage than 

 in Antedon. The phagocytes occur during the metamorphosis, as in Antedon. 

 This matter is more fully discussed on page 13. 



The hydrocoel is assuming the shape of a horseshoe, open downwards; 

 there are thus apparently two vesicles in some sections (plate xv, figures 

 7, 8) ; its walls consist of high, cyhndrical cells, forming a regular columnar 

 epithehum. The parietal canal has developed into a large sac with a lining 

 of flattened, endothehal cells. It has a large forward prolongation, reaching 

 to just below the suctorial disk, where a httle widening may occur (plate xv, 

 figm-e 11). At its lower end it is constricted into a long, narrow pore-canal, 

 which may have an exterior opening, the hydropore, situated between the 



