ILLUSTRATIONS. 89 



PLATE XV. 



(All the figures represent Isomclra nripara.) 



Flo. 1. Sagittal, lonp;ilu(linal section of an cmbrj'o, in which the division of the archenteron into the rcrlomic 

 vesicle and the entero-hydrocu 1 lias been coin|)leted. The niescncliyine rells have not yet 

 filled the blastocnM cavity completely in the lower part. The concavity on the left (ventral) 

 side rei)rescnts the beginning formation of the vestibnlary invagination. X1C5. 



Fig. 2. Part of the entoderm from the section represented in plate xiv, figure 0. X535. 



Fifi. 3. Part of the ectoderm from the .section represented in plate xiv, figure 4. X535. 



Fio. 4. Transverse section of an embryo in which the parietal canal does not open outwards; it is not 

 distinctly apically elongated, being present in only five sections upon the whole, and thus 

 represents a comparatively yoimger stage of development than figure 5, as is also seen from the 

 fact that the vestibular}- invagination is quite indistinct. In the mesoderm a fairly large yolk 

 globule {i/. g.) is seen. X 105. 



Fio. 5. Transverse section of an embrj-o, showing the parietal canal opening outwards; the vestibulary 

 invagination is a slight but distinct concavity on the ventral side. X1C5. 



Figs. 6, 7. Two transverse sections of an embryo. The two sections figured are separated from one another 

 by five .sections. 

 Figure C is a section from the posterior end, showing the two enterocccl vesicles surrounding the 

 entoderm separated from one another in the dor.sal and ventral midline by a thin wall, the 

 mesentery. The entoderm with a mere indication of a lumen, otherwise filled with a 3'olk- 

 mass, containing nuclei. 

 Figure 7. More anterior section, showing the hydroco?! cut in two places. The pore canal is indis- 

 tinct in this series; it is represented only by a compact mass of cells, no lumen being discernible. 

 The left enterocal has disappeared in this section. Glandular cells are fairly numerous in 

 the ectoderm. X 165. 



Fig. 8. Transverse section, corresponding to that represented in figure 7; shows a very large globule of 

 yolk and some smaller ones in the mesoderm. In this embrj'o there is a rather large lumen 

 in the entoderm, in which a fine granulated mass is seen, probably dissolved yolk-mass. The 

 extraordinary outline of this figure, which has even been sUghtlj' corrected, is due to the fact 

 that the embryo has been irregularly compressed by the mutual pressure of the embryos 

 within the marsupium. X165. 



Fig. 9. Longitudinal, frontal section, showing the parietal canal opening outwards through the hydropore. 

 The hydrocccl is visible in this section only as a small mass of nuclei lying between the left 

 (oral) ccelom and the pore canal. The vibratile bands are distinct. X165. 



Figs. 10, 11. Two longitudinal, frontal sections of an embrj'o. 



Figure 10. The more dorsal of the two, showing the beginning formation of the chambered organ 

 as prolongations from the right, aboral ccelom. The nervous system is beginning to develop. 

 Figure 11. A more ventral section, separated from that figured in figure 10 by sixteen sections. 

 It shows the anterior prolongation of the parietal canal; no opening of the pore canal could 

 be discerned in this series. In the aboral ca'lom the vertical mesentery has been formed. 

 The concavity of the anterior end of the embrj'O is the suctorial disk. The vibratile handsale 

 distinct. X 165. 



PLATE XYI. 

 (All the figures represent Isomeira Hni>ara. All X105.) 



Fig. 1. Transverse section of an embryo showing the formation of the parietal canal as an outgrowth from 

 the hj'droctcl. The outline of the figure slightly corrrcteil. 



FlOS. 2 to 5. From a frontal, longitudinal series of sections of a fully formed larva. Figure 2 is the more 

 ventral of the sections; nine sections lie between figures 2 and 3, four sections between figures 

 3 and 4, twelve .sections between figures 4 and 5. 

 Figure 2 is in the level of the outer opening of the vestibulary invagination. 



Figure 3 is at the bottom of the invagination, which is seen to be deeper in its anterior part The 

 different arrangement of the glanihilar cells and the nuclei around the invagination in figures 

 2 and 3 is to be noted. The notch in the anterior end in figure 2 is the suctorial disk. In 

 figure 3 the parietal canal and the apparently obliterating pore canal are seen. Parts of 

 the nervous system appear in the anterior end of this figure; in the posterior end some yolk- 

 globulcs are seen. 

 Figure 4 shows the hydroccrl, from which the primary radial canals are about to develop. The dark 

 mass above is a group of glandular cells from the bottom of the vestibulary invagination 

 in transverse section. In the posterior end is seen a pair of large yolk-globules. The groups 

 of nuclei in the ectoderm of this and the following figure indicate the vibratile bands. 

 The cilia are indistinct in this series on account of the egg-membrane lying very close to 

 the epidermis. 

 Figure 5 goes through the middle of the entoderm, showing the indistinctly limited lumen in the 

 middle of the mass of yolk-cells. The series of narrow luniina above the vertical mesentery 

 are due to the decalcified young stalk-joints. 



