124 EXPLAXATIOX OF THE PLATES. 



Plates I., II. Embryology of AsTERACAXTniox berylixus Ag. 



PI. I. Figs. 22-28, PI. ir. Figs. 2-19, ScypbLstoraa stage; PI. II. Fig.s. 20-24, Tomaria stage j PI. II. Figs. 25-28, Brachina stage. 



PLATE I. 



Fig. 1. A mature egj^, surrounded by spermatic particles, 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 tlie 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 sliows 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 (he yolk divided into two united ellipsoids, the whole yolk rotating slowly, always in one 

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



Fig. 7. Tlie two segments of the yolk have entirely separated. The Richtungsblaschen are likewise iso- 

 lated at one [kjIc 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. Diflerent view of Fig. 10, showing the position of the segments. 



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



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 op|)osite sides of the envelope. 



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. 'J 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. 11). These si)heres 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 

 s|)heres, which are now somewhat polygonal, owing to their pressure upon each othei . 



Fig. 21 represents an egg ten hours after segmentation; the spheres are still more polygonal; the rotation 

 of the yolk is (juite rapid, and the embryo is ready to break through 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 nuich thickened at one pole (a), while the spheres of segmentation are 

 somewhat indistinct. 



Fig. 23. The embryo has been slightly flattened at the ]wle (n), where the wall is thickest; the planula, 

 if we may so call it in its jiresent condition, reached this stage at the end of alK)Ut eleven hours. 



Fig. 21. 'Hie wall of the flattened jtoie has been pressed in so as to curve sliirhtly inward (n). 



Fig. 2.'). The dej)ression (ri) has become nuich deeper, and the spheres of segmentation have entirely dis- 

 appean-d. tw.'Iv.« hours after lecun(kition. The depression at a ;issumes here somewhat the asjK'ct of 

 a digestive cavity. 



Fig. 2(5. Seventeen hours after fecundation ; the embryo has lost its spherical sha|>e and has l)ocomc 

 somewhat pear-.shajM-il ; a transver.se section is still circular. Tlie depression made by the thickened 

 walU has incirased in de|)th; the opening («) performs the functions of a mouth and anus; <; indi- 

 cates the bottom of the digestive cavity. 



