310 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



Mortensen recalled that Barrens has recorded a similar unintentional experiment 

 with the larvae of Antedon, the result being the same the development of the internal 

 organs goes on normally despite the abnormal position. Only it does not appear 

 that Barrois has seen them develop into true pentacrinoids. Seeliger also has seen 

 abnormal embryos developing their tentacles, although the closure of the vestibulum 

 had not taken place even less so than in the cases observed by Barrois and Morten- 

 sen there being in the cases observed by Seeliger no covering up at all of the vestib- 

 ulary invagination, so that the tentacles are at once free. Seeliger did not see these 

 abnormal embryos develop into pentacrinoids. As was pointed out by Seeliger, the 

 crinoid embryo with tentacles developed on the ventral side described and figured 

 by Busch must have been such an abnormal embryo. 



The main point, or at least the most conspicuous point, in the transformation 

 of the embryo from free swimming to fixed is the closure of the vestibulum. This 

 proceeds in the same way as it does in Antedon. 



The glandular character of the cells of the vestibulum is indicated by the fact 

 that they retain the hematoxylin as strongly as do the glandular cells of the outer 

 ectoderm, but it disappears with the closure of the vestibulum. Also in the epidermis 

 itself the glandular cells disappear. This is correlated with the intermixing of the 

 ectoderm with the mesenchyme, which process has been completed by the time the 

 larva has attached itself. Some few cells may still remain attached to the surface 

 by means of more or less branching prolongations. That these "multipolar" cells 

 are the remnants of the ectoderm cells can hardly be doubted, for they are not found 

 in the pentacrinoid stage. 



The suctorial disk loses its special histological character as soon as fixation has 

 taken place. The same is the case with the larval nervous system. 



The exact time when the hydrocoel ring closes has not been ascertained, but it 

 takes place during the stage of transformation from the free swimming larva to the 

 young pentacrinoid, and the important organs, the pore canal and the stone canal, 

 are formed also during this period. 



The pore canal was seen first in the four-day-old free-swimming embryo. It 

 still has no exterior opening. Its appearance in the specimen as a small ring lying 

 within a more spacious lumen must be due to some contraction during preservation. 

 The stone canal is quite short. In one section a thickening is seen in the left oral 

 mesentery, an ovoid mass of cells lying in the space between the epithelium of the 

 oral and aboral coelom and separating them from each other. Mortensen says it 

 seems certain that this corresponds to the primary genital gland fouud in Aniedon 

 by Russo, a rudimentary organ which soon disappears, but which, according to Russo, 

 is of great morphological importance, being homologous with the genital gland of the 

 holothurians, while the axial organ of the crinoids is a thing apart, and not at all 

 homologous with the ovoid gland of other echinoderms. 



Russo in Antedon found the first traces of this gland in pentacrinoids which had 

 already been attached for some five or six days and already had a long stalk. In 

 Tropiometra it appears somewhat earlier. Mortensen was unable to see this gland 

 in decalcified and stained pentacrinoids of Tropiometra, though it was easily observ- 

 able in unsectioned pentacrinoids of Compsometra serrata and of Isometra vivipara. 

 Mortensen said that the formation of the primary tentacles needs no description. 



