A MONOGRAPH OF THE EXISTING CRINOIDS 307 



sen says, this certainly needs confirmation, as it seems doubtful whether so conspicuous 

 a difference would exist between closely related species. 



The second band apparently ends abruptly on the sides of the vestibulary invagi- 

 nation, but on closer observation it may be seen to continue along the borders within the 

 vestibulum, around its posterior end, although in the specimen figured this could not 

 be distinctly ascertained. The first band is pushed slightly upward, the third slightly 

 downward, by the vestibulary invagination. The posterior ciliated band lies in a 

 slight depression. The anterior or apical pit is not so distinctly circumscribed as in 

 Antedon. 



The vestibulum, which begins as a flattening of the ventral side of the embryo at 

 the age of 12 hours, now forms a distinct invagination of broad oval outline. 



The ectoderm in embryos 16 to 20 hours old is distinctly limited toward the 

 mesenchyme, but from the age of 25 hours onward no limit can be seen. In sections 

 stained with hematoxylin elements are seen in the ectoderm that stain very strongly 

 and look like glandular cells. Mortensen says that although the preservation is not 

 quite satisfactory, he has no doubt that these elements correspond to the "yellow 

 cells" of the Antedon larva, which are also supposed to be of glandular nature. In 

 the vestibulary invagination the ectoderm stains very strongly in hematoxylin, a 

 feature which does not depend alone on the fact that the nuclei are here much more 

 numerous than in the other parts of the ectoderm. Possibly this indicates a glandular 

 character of the cells of the invagination. Bury mentioned the same feature in 

 Antedon, but pointed out that they lose their color much more readily in acidulated 

 alcohol than do the glandular cells of the ectoderm. Bury found these deeply staining 

 cells only In the anterior, deeper, part of the vestibulary Invagination. In the 

 Tropiometra larva the cells have this character throughout the whole length of the 

 invagination. 



In a specimen 25 hours old Mortensen observed very distinctly a feature described 

 by Seeliger in Antedon, that is, that the ectoderm cells secrete between themselves an 

 intercellular substance, this process being the beginning of the transformation of the 

 ectoderm which ultimately results in the complete intermingling of the ectoderm cells 

 with the mesenchyme, so that in the later stages of development there is no separate 

 ectoderm layer. The details of this process could not be followed hi Tropiometra, but 

 since the first stage of the process and also the end result are the same in Tropiometra 

 as in Antedon, that is evidently no reason to doubt that the whole of the process is 

 identical with that obtaining in Antedon, as described by Seeliger. 



In an embryo 40 hours old a depression is visible just in front of the vestibulary 

 invagination that evidently represents the suctorial disk by means of which the larva 

 attaches itself. It is seen very indistinctly except in sections. The anterior ciliated 

 band passes uninterruptedly between the disk and the invagination, as is also the case 

 in Antedon. 



The ciliated bands generally appear in sections as slightly concave structures; 

 sometimes, however, they are distinctly convex, while at other times they appear to be 

 nearly flat. The differences are evidently due to differences in preservation. 



The nervous system, first seen by Bury in the Antedon larva and described in detail 

 by Seeliger, appears to exist also In the Tropiometra larva in at least the same degree 

 of differentiation, or probably even somewhat more strongly developed. In the an- 



