88 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



Fio. 9. Longitudinal, frontal section through an embryo of the same age. Shows the large size of the parietal 



canal and its anterior prolongation. 

 FIG. 10. Longitudinal frontal section through another embryo of the same age, showing the opening of the 



pore canal. In figures 8, 9, 10 glandular cells are rather strongly developed in the anterior end. 



The embryos represented in this plate all lie within the egg-membrane. 



PLATE XII. 



(All figures of Compsometra serrata.) 

 FIGS. 1, 2. Fully formed embryo, ventral view (1) and side view (2); the latter from a specimen mounted 



in balsam and stained with paracarmin. The dark line inside the contour-line represents 



the layer of nuclei. X 150. 

 FIGS. 3, 4. Two stages of the formation of the skeleton. The small calcareous grain inside the basal circle 



in figure 3 represents the first infrabasal; in figure 4 the 4 infrabasals are well developed. X270. 

 FIG. 5. Young Pentacrinoid, decalcified; vestibulum still closed; shows the primary gonad, pore canal, 



and chambered organ. The pharynx is distinctly compressed. The intestine and rectum 



fully formed. The stone canal could not be traced very distinctly, but the form shown in the 



figure apparently is correct. X200. 



FIG. 6. Pentacrinoid showing the young costal and axillary. X 105. 

 FIG. 7. Slightly older Pentacrinoid; the anal plate is seen partly overlying the adjoining oral. The long 



tentacles with their spicules are noticeable, as in the preceding figure. X90. 



PLATE XIII. 



(All figures of Compsomelra serrata.) 



FIG. 1. Young Pentacrinoid, the vestibulum just opened. X105. 



FIG. 2. Slightly older stage of Pentacrinoid; the radiala and the anal plate have appeared. X105. 



FIG. 3. Pentacrinoid, later stage; the arms have begun to form. The oralia have been separated from the 

 basalia. X43. 



FIG. 4. Pentacrinoid, further stage of development; the cirri have just begun to appear. The radialia are 

 laterally in contact and the oralia are widely separated from both radialia and basalia. The 

 pinnules have not yet begun to form. X43. 



FIG. 5. Nearly fully formed Pentacrinoid. Cirri distinct. The first pinnules have been formed. The 

 large anal plate is seen at the base of the high anal tube, through the walls of which is seen 

 the oral of the anal interradius. The lower end of the stalk has been placed alongside the real 

 of the stalk in order to have it all included within the space of the plate. X43. 



PLATE XIV. 



(All the figures represent Isometra vivipara. All X165.) 



FIG. 1. Young cleavage stage. The egg-membrane has been indicated in this figure alone, but has been 

 omitted in all the following figures. 



FIG. 2. Later stage of cleavage. The nuclei have augmented considerably and the grouping near the surface 

 and in the middle is beginning to be distinct. 



FIG. 3. More advanced cleavage stage, showing more distinctly the arrangement of the nuclei near the 

 surface and in the middle of the yolk-mass. 



FIG. 4. The ectoderm and entoderin have become differentiated, cell limits being distinct. At one end, 

 probably the anterior end of the embryo, a space has appeared between ectoderm and entoderm, 

 filled with groups of yolk-spherules. A large cavity is formed in the archenteron. The part 

 of the ectoderm marked * has been represented more magnified in plate xv, figure 3. 



FIQ. 5. Beginning formation of the cavity of the archenteron. The ectoderm cells not yet distinctly 

 limited, although the nuclei have augmented beyond the stage represented in figure 4. The 

 concavity on the right side is probably due simply to mutual pressure of the embryos lying 

 in the marsupium. 



FIG. 6. Sagittal longitudinal section of an embryo, showing beginning differentiation of the archenteron 

 into ca'lomic vesicle (c. v.), entoderm (ent.), and hydroccel (h.); mesenchyme cells are seen 

 lying in the blastocoel cavity. Part of the entoderm (at the mark *) has been represented 

 more magnified in plate xv, figure 2. 



FIG. 7. Frontal, longitudinal section of an embryo, with the division of the archenteron completed. The 

 coelomic vesicle has separated into the two enterocal vesicles, right and left (r.c. and I.e.). 

 The downward prolongation of the entoderm marks the former connection between the ento- 

 derm and the ccelomic vesicle. 



FIQS. 8, 9, 10. Three frontal longitudinal sections of an embryo, with the co?Iomic vesicle separated from 

 the entoderm, but the left and right enterocoelic vesicles are still connected by a narrow trans- 

 verse canal. The sections have been somewhat obliquely directed and the three figures com- 

 bined correspond to figure 7. 



FIG. 11. Frontal, longitudinal section of an embryo, showing the separation of the archenteron in an upper 

 part (entero-hydrocoel) and a lower part (coelomio vesicle) not yet completed. 



