24 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



1. THE EMBRYOS. 



The egg is, judging from the ripe eggs within the pinnulae of the preserved 

 specimens, rather small, only about 0.25 mm. The eggs remain attached in 

 clusters to the pinnules around the ovarian opening, as in Antedon. The egg- 

 membrane does not appear to have any structural peculiarities, but is simple, 

 as in Antedon, serving only as means of attachment, not as a floating appa- 

 ratus as in Tropiometra. It appears to be very spacious, affording ample 

 room, so that the embryo is not globular, as is the case in Antedon, but while 

 still lying within the membrane has its natural, elongate shape a circum- 

 stance very fortunate for the study, the orientation of the embryo by section- 

 ing being fairly easy, whereas the Antedon embryos are very difficult to orient 

 while they remain within the egg-membrane. 



From the material in hand it is evident that the embryos remain within 

 the egg-membrane until fully formed and ready to attach themselves and 

 transform into Pentacrinoids. It may then be concluded with a fair degree 

 of certainty that they have only a very short free-swimming period, and this 

 supposition is strengthened by the fact that the Pentacrinoids were found 

 very close to the localities where the Crinoids were obtained. 



The cleavage, the formation of the archenteron, and the formation of 

 enterocoel and hydroccel has not been observed. The youngest stage repre- 

 sented is that figured in plate xi, figures 1 to 4. 



The hydroccel has occupied its place on the ventral side and the parietal 

 canal is about to separate from it (figure 3, plate xi). The two enterocoel 

 vesicles have separated completely from one another and occupied their 

 usual position, the left at the posterior end, the right at the dorsal side of the 

 entoderm. Unfortunately this stage is too far progressed to show whether 

 the entoderm sends out posterior prolongations to embrace the middle part 

 of the enterocoel vesicle, as is the case in Antedon but not in Tropiomefra. 

 The anterior end of the right enteroccel is more thick-walled than the rest 

 of the vesicle (plate xi, figures 1 and 4) ; it would appear that this has some- 

 thing to do with the formation of the chambered organ. I was at first inclined 

 to think that it might be the parietal canal which was being formed in this 

 case from the right enteroccel vesicle instead of the hydroccel, which would 

 certainly be astonishing. But at length the question was settled by the 

 oblique sections, from which figure 3 has been drawn, which proves that 

 the formation of the parietal canal is in conformity with what obtains in 

 Antedon and Tropiometra and what must evidently be regarded as the rule 

 in Crinoids. 



The vestibulary invagination and the suctorial disk have begun to form, 

 as seen in figure 1 and, in a slightly more advanced stage, in figure 4. Also the 

 vibratile bands have begun to develop, as is evident from the arrangement 

 of the cells in the ectoderm, especially in figure 4. There are no glandular 

 cells seen in the ectoderm. The apical pit I have not found distinct. As in 



