8 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



larger at one pole of the embryo (plate i, figure 6). This inequality, how- 

 ever, soon disappears completely and I have been unable to ascertain whether 

 the invagination takes place at the pole, where the larger cells occurred. 



When the embryo is about 2J^ hours old the formation of the entoderm 

 begins. It starts with the wandering into the cavity of the blastosphere of 

 several cells; these cells lie loosely in the cavity and look like mesenchyme 

 cells (plate i, figures 7, 8, 9), which, however, they are not. It appears that 

 they come irregularly from different places of the blastosphere ; thus in plate 

 i, figure 8, are seen in one place two elongated cells, in another place one 

 cell similarly elongated and protruding beyond the level of the other cells 

 into the segmentation cavity, apparently being about to wander in ; however, 

 I can not ascertain this beyond doubt. When the cavity is nearly full of 

 these cells, the typical invagination takes place, and the loose cells now 

 arrange themselves regularly at the upper end of the invagination (plate n, 

 figures 2 and 3; plate i, figure 10). Probably the formation of the mesoderm 

 cells starts again from these cells, but at one time all the formerly loose cells 

 have joined the entodermal invagination. 



The formation of the entoderm thus differs considerably from that of 

 Antedon, where (according to Seeliger, who has made a most careful study 

 of this process) no such wandering of free cells into the blastoccel takes place 

 before the invagination. However, it must be mentioned that in one case 

 I found the invagination starting before any loose cells had wandered into 

 the segmentation cavity (plate n, figure 1). In this case the formation of 

 the entoderm thus proceeds as in Antedon. It is rather startling to find that 

 there can be so great variation within the same species in so important a 

 process as the formation of the entoderm. 



The ectoderm cells in the oral half of the embryo have their inner ends 

 turned upwards, making a very characteristic arrangement (plate n, figures 

 1, 2, and 3) ; it looks as if they were pushed upward by the invagination. The 

 little space left by the archenteron, together with the considerable elongation 

 of the ectoderm cells at this stage, accounts for this peculiar feature. 



In the lower end of the archenteron, near the blastopore, the cells are 

 quite low; in the upper, wider part they are high and cylindrical. The cavity 

 of the archenteron is very narrow and makes a characteristic curve in the 

 upper part (plate 11, figures 2 and 3; plate i, figure 10). The blastoporus is a 

 small, round opening, not an elongated slit, as in Antedon. 



The gastrula is fully formed about 5 hours after the fertilization. In Antedon 

 this stage is reached (according to Seeliger) about 16 hours after fertilization, 

 but according to Barrois and Bury it is not reached until 24 hours after 

 fertilization, this discrepancy being evidently due to the fact that Seeliger 

 worked on Antedon adriatica, while Barrois and Bury worked on Antedon 

 mediterranea, as pointed out by A. H. Clark. 12 



12 A. H. Clark, A new European Crinoid, Proc. U. S. Nat. Mus. 38, 1910, p. 329. 



