84 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



FIG. 5. Longitudinal section through an embryo 8 hours old. The archenteron is nearly divided into an 

 upper (anterior) and a lower (posterior) part through a median constriction. The blastoeosl 

 cavity has been completely filled up by mesenchyme cells. Also the space below the archen- 

 teron has been filled up by mesenchyme cells, which are evidently derived from the posterior 

 end of the archenteron. The cell-limits of the mesenchyme cells are not represented exactly 

 in every detail. As regards the entodenn, it has been combined from 2 to 3 sections follow- 

 ing directly after one another, both sides of the median constriction not being distinct in 

 one section. 



FIQ. 6. Longitudinal section through an embryo 10 hours old. The division of the archenteron into an 

 anterior and a posterior vesicle has been completed. This embryo has been rather strongly 

 contracted. 



FIG. 7. Longitudinal section through another embryo 10 hours old, in which the division of the archenteron 

 is still incomplete. The widening of the upper vesicle may possibly represent the first beginning 

 of the hydroccel. The preservation of this specimen was not very good; this accounts for the 

 condition of the mesenchyme. 



Fio. 8. Section through an "embryo" 6 hours old. The two large, cell-like bodies possibly represent a 

 parasitic organism. The small spots filling the rest of the space within the membrane have 

 the appearance of very small nuclei. Possibly these represent minute organisms which have 

 devoured the cells of the embryo. 



Fid. 9. Transverse section through an embryo 12 hours old, showing the constriction of the coelomic vesicle, 

 about to divide. 



PLATE III. 



(All figures of Tropiometracarinata. All X290). 



FIQ. 1. Longitudinal section through an embryo 10 hours old, probably abnormal. The constriction of the 

 archenteron appears to proceed from only one side. 



FIG. 2. Longitudinal, frontal section through an embryo 12 hours old. The division of the archenteron is 

 complete, but the former connection is still partially visible. The posterior or coelomic vesicle 

 has begun to form the right and left enteroccel. 



FIG. 3. Longitudinal, somewhat oblique section through an exceptionally large embryo 12 hours old. The 

 anterior vesicle has a very distinct widening, representing the beginning of the hydroccel. 

 There is an indication of an apical pit. Not well preserved, so that the mesenchyme could not 

 be represented in a more detailed way. 



FIG. 4. Longitudinal, frontal, but somewhat oblique section of an embryo 12 hours old. The ccelomic 

 vesicle has nearly completed its division into a right and a left vesicle, the right and the 

 left enteroccel; they have begun to occupy their later position, the left at the posterior end, 

 the right at the dorsal side of the embryo. The arrangement of the nuclei of the ectoderm 

 begins to show the ciliated bands and the outline of the posterior end shows the characteristic 

 depression occupied by the posterior ciliated band. 



FIGS. 5, 6. Two longitudinal, sagittal sections of an embryo 16 hours old; two sections lie between those 



two figured. 



In figure 5 is seen the vestibulary invagination indicated by the numerous nuclei along the slightly 

 flattened ventral (in the figure left) side. The ciliated bands are distinctly seen by the 

 grouping of the nuclei. The ccelomic vesicle is completely divided and the two enterocosls 

 have occupied their normal position, the left at the posterior end, the right along the dorsal 

 side of the embryo covering the entoderm. The hydrocrel has been separated from the rest 

 of the anterior vesicle, which now represents the future entoderm alone. The hydroccel has 

 occupied a position on the ventral side, just under the vestibulary invagination. 

 In figure 6 the hydrocoel is seen to divide off a small vesicle at its anterior end, the parietal canal. 

 It is hardly so distinct in the section as shown in the figure, but is combined from this and 

 the preceding section. The lumen of the entoderm is not seen in this figure, which repre- 

 sents a slightly lateral section. The ectoderm is seen to be thickened in the anterior end of 

 the embryo. 



FIGB. 7, 8, 9, 10, 11. From a series of transverse sections through an embryo 20 hours old; figure 7 is the 

 foremost, figure 11 the hindmost; figure 7 is from the anterior end, in the region of the ves- 

 tibulary invagination. The pariet.-J canal is seen in this section and in 4 more sections; it is 

 completely separated from the hydrocoel. The vestibulary invagination gradually disappears 

 towards the posterior end, but is still visible in figure 10. The hydroccel, as shown in figures 

 8 to 10, has the shape of a curved tube; it is not yet a closed ring. The relative position and 

 shape of the enterocoels appear from figures 8 to 11. There are 5 sections between figures 7 

 and 8; figure 9 follows directly after 8; between 9 and 10 there is 1 section, and 3 sections 

 between 10 and 11. 



