124 • EXPLANATION OF THE PLATES. 



Plates I., II. Embryology of Asteracantiiion berylinus Ag. 



PI. I. Figs, 22-28, PI. II. Figs. 2-19, Scyphistoraa stage; PI. II. Figs. 20-24, Tomaria stage ; PI. II. Figs. 25-28, Brachina stage. 



PLATE I. 



Fig. 1. A mature egg, surrounded by spermatic particles, soon after the artificial fecundation. The egg 

 has assumed a spherical shape, and contains the germinative vesicle and dot. Tliere 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 Richtungsblaschen are visible on one side 

 of the yolk. 



Fig. 6 shows the yolk divided into two united ellipsoids, the whole yolk rotating slowly, alway.s in one 

 direction, from right to left. The Richtungsblaschen are at one pole of the axis of segmentation. 



Fig. 7. The two segments of the yolk have entirely separated. The Richtungsblaschen 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. n. Different view of Fig. 10, showing the position of the segments. 



Fig. 1 2. 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 envelope. 



Fig. 14. This view of the egg shows the tendency of the spheres of segmentation to airange themselves 

 on the circumference. 



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



Fig. 16. '.. he 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. 1 7. 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 distinj;uished 



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 ttrough the outer membrane ; the shell 

 envelope 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 through- 

 out, but has become very much thickened at one pole (a), while the spheres of segmentation are 

 somewhat indistinct. 



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 dis- 

 appeared, twelve hours after iecundation. The depression at a assumes here somewhat the aspect of 

 a digestive cavity. 



Fig. 26. Seventeen hours after fecundation ; the embryo has lost its spherical shape and has become 

 somewhat pear-shaped ; a transverse section is still circular. Tlie depression made by the thickened 

 walls has increased in depth; the opening («) performs the functions of a mouth and anus; Vindi- 

 cates the bottom of the digestive cavity. 



