EXPLANATIONS OF PLATES. 



LIST OF ABBREVIATIONS USED. 



a. anal plate. gl. s. glandular sac. 

 a. c. aboral coelotn. h. hydrocoel. 



an. anal opening. h. r. hydrocoel ring, 



an. c. anal cone. i. intestine, 



arm. c. arm coelom. i. b. infrabasal. 



arch, archenteron. i. t. interradial tentacle, 



a. t. anal tube. 1. c. left coslomic vesicle, 



ax. axial organ. m. mouth, 



axill. axillary. m. 3. mesenchyme cells. 



b. basal plate. n. nervous system. 



c. cirrus. o. oral plate, 

 c. b. ciliated band. o. c. oral ccelom. 



c. gr. ciliated groove. oe. esophagus. 



ch. o. chambered organ. o. v. oral valve. 



co. costal. p. pinnule. 



c. v. coelomic vesicle. p. c. parietal canal. 



d. dorsal. po. hydropore. 

 e. h. entero-hydrocoel. po. c. pore canal, 

 ent. entoderm. pr. g. primary gonad. 



gl. c. glandular cells. pr. t. primary tentacle. 



pt. apical pit. 

 r. rectum, 

 rad. radial plate, 

 r. c. right coelomic vesicle, 

 r. n. ring nerve, 

 r. v. radial vessel. 



s. sacculus. 

 s. d. suctorial disk. 



st. stomach, 

 st. c. stone canal, 

 et. j. stalk-joint, 

 s. t. pi. supplementary terminal 



plate. 



t. tentacle. 



t. pi. terminal stem-plate, 

 t. v. tentacle vessel. 



v. ventral. 



v. m. vertical mesentery, 

 vst. vestibulum. 

 y. g. yolk globule. 



The sections are all 5 fi thick. 



PLATE I. 



FlQ. 



(All figures of Tropiometra carinata.) 



1. An egg immediately after liberation from the follicular membrane. X200. 



Fia. 2. An egg just fertilized. The formation of the egg-membrane has started at one pole, whence it 

 spreads all over the egg. X200. 



Fia. 3. First cleavage stage. The fully formed egg-membrane is represented in this figure and in figures 

 4 and 5. X200. 



Fia. 4. The 4-cell stage. The different size and shape of the cells in this figure is due to the fact that they 

 are not seen directly from above; in reality they are exactly alike. X200. 



Fia. 5. The 32-cell stage. X200. 



FIQ. 6. Optical section through a young blastula, showing the different size of the cells. X200. 



FIG. 7. Section through an embryo 2*A hours old, showing cells lying loosely in the blastctcoel. At the lower 

 side of the figure is seen one cell in the act of wandering in. X290. 



Fia. 8. Another section from the same series, showing a considerable number of cells which have wandered 

 in. At the right side of the figure is seen 1, at upper side 2 cells, probably in the act of 

 wandering in. That on the right side has its nucleus in division. X290. 



FIG. 9. Section of another embryo 2} hours old. X290. 



FIQ. 10. Transverse section of an embryo in the gastrula stage. 5 hours old. X290. 



In figures 7 to 10 the egg-membrane is represented as seen in the sections, only in a slightly dia- 

 grammatic way. 



PLATE II. 

 (All figures of Tropiomeira carinata. All X290.) 



FIG. 1. Optical section through an embryo 5 hours old in the beginning of the gastrula stage. No cells 

 have wandered into the blastocosl before the invagination in this specimen. 



FIG. 2. Longitudinal section through an embryo in the gastrula stage, 5 hours old. The figure is partially 

 composed from different sections, none of them being sufficiently well preserved to show all 

 the details. At the aboral end of the archenteron the cells are not regularly arranged, these 

 cells probably representing the beginning formation of the mesenchyme cells. 



FIG. 3. Optical longitudinal section through an embryo in the gastrula stage, 5 hours old. Only two mesen- 

 chyme cells at the aboral end of the archenteron. The curvature of the cavity of the archen- 

 teron is seen in this figure, as also, less distinctly, in figure 2. 



FIG. 4. Longitudinal section through an embryo 6 hours old, immediately after liberation from the egg- 

 membrane. The archenteron is completely separated from the ectoderm and the blastopore 

 has disappeared. The formation of the mesenchyme cells from the anterior end of the archen- 

 teron is in rapid progress. The nuclei of the entoderm and mesoderm are distinctly larger 

 than those of the ectoderm. (In some few places on the right of the section figured the 

 cell-limits were not quite distinct, the figure being thus far slightly reconstructed.) 



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