(*) 



EXPLANATION OF THE PLATES. 



In all the figures of the Brachiolaria, Plates I.-IV., the 

 attitude which has been given to them is not a natu- 

 ral attitude. This has been done purposely, for the 

 sake of making the comparison with the memoirs of 

 Miiller easier. The only figure of a Brachiolaria 

 which is in its natural attitude, is that of PL VIII. 

 Fig. 8. The young Brachiolaria does not, however, 

 move with the anal part below, till the latter is loaded 

 down by the development of the Starfish, and we see 

 them swimming about, before that time, almost in every 

 ssible attitude. 



Plates I., II. Embryology of Asteracanthion 

 berylinus Ag. 



PI I. Figs. 22-28, PI. II. Figs. 2-19, Scyphistoma stage ; PI. II. Figs. 

 20-24, Tornaria stage ; PI. II. Figs. 25-28, Brachina stage. 



PLATE I. 



Fig. 1. A mature egg, surrounded by spermatic parti- 

 cles, soon after the artificial fecundation. The egg 

 has assumed a spherical shape, and contains the 

 germinative vesicle and dot. There is no trace of any 

 interval between the yolk and the outer envelope. 



Fig. 2. The germinative vesicle has disappeared, but 

 the germinative dot remains. 



Fig. 3. The germinative dot is no longer visible ; the 

 yolk has contracted, and is separated by a slight 

 space from the outer envelope. The egg has all 

 the appearance of having already gone through the 

 segmentation ; the whole yolk being made up of 

 small spherical cells, resembling very minute spheres 

 of segmentation, although the segmentation has not 

 yet commenced. Two hours after fecundation. 



Fig. 4 shows the first trace of segmentation, consisting in 

 a depression on one side of the yolk. 



Fig. 5. The yolk has become flattened on opposite poles; 

 the Eichtungsblaschen are visible on one side of the 

 yolk. 



Fig. 6 shows the yolk divided into two united ellipsoids, 

 the whole yolk rotating slowly, always in one direc- 

 tion, from right to left. The Eichtungsblaschen are at 

 one pole of the axis of segmentation. 



Fig. 7. The two segments of the yolk have entirely 

 separated. The Eichtungsblaschen are likewise iso- 

 lated at one pole of the axis of segmentation. 



Fig. 8. First trace of a further segmentation ; one half 

 of the yolk is partially divided. 



Fig. 9. The two yolk segments are about to separate 

 into four. 



Fig. 10. The four yolk segments are all distinct, and 

 almost transformed into regular spheres. 



Fig. 11. Different view of Fig. 10, showing the position 

 of the segments. 



Fig. 12. The yolk about to separate into eight spheres. 



Fig. 13 shows eight spheres of segmentation, all of which 

 are more or less spherical ; the spheres are arranged 

 in two clusters of four, on opposite sides of the en- 

 velope. 



Fig. 14. This view of the egg shows the tendency of 

 the spheres of segmentation to arrange themselves on 

 the circumference. 



Fig. 15. The yolk is divided into sixteen spheres. 



Fig. 16. The shell of segmentation is composed of 

 thirty-two spheres; owing to the position from which 

 the egg is viewed, only half the shell of segmentation 

 is visible. 



Fig. 17. The thirty-two spheres are again subdivided. 



Fig. 18. The spheres of segmentation are still smaller 

 than in the preceding figure. 



Fig. 19. These spheres have become so small, that the 

 walls of the spherical shell formed by them can be 

 readily distinguished. 



Fig. 20. The walls have become still more distinct in 

 consequence of the close packing of the small spheres, 

 which are now somewhat polygonal, owing to their 

 pressure upon each other. 



Fig. 21 represents an egg ten hours after segmentation ; 

 the spheres are still more polygonal ; the rotation of 

 the yolk is quite rapid, and the embryo is ready to 

 break through the outer membrane ; the shell en- 

 velope is very distinct from the inner contents, and 

 has a uniform thickness. 



Fig. 22. An embryo after its escape from the egg; the 

 wall is no longer of the same thickness throughout, 

 but has become very much thickened at one pole (a), 

 while the spheres of segmentation are somewhat indis- 

 tinct. 



Fig. 23. The embryo has been slightly flattened at the 

 pole (a), where the wall is thickest ; the planula, if 

 we may so call it in its present condition, reached 

 this stage at the end of about eleven hours. 



Fig. 24. The wall of the flattened pole has been pressed 

 in so as to curve slightly inward (a). 



Fig. 25. The depression (a) has become much deeper, 

 and the spheres of segmentation have entirely disap- 



