EMBRYOLOGICAL MONOGRAPHS. 
PaAw Bs Velie 
Development of ASTEROIDEA, continued. Figures from ALEXANDER AGasstz. 
a. Anus. J’. Surface warts at the base of the odd brachiolar 
2. Dorsal or water pore, madreporie opening. arm (f!'). 
c. Alimentary canal. m. Mouth. 
d. Digestive cavity. nm’. Pistol-shaped oral pouch of esophagus. 
e!.. Median anal arms of Brachiolaria. m'. Anal pouch of cesophagus. 
e!!, Dorsal anal arms of Brachiolaria. o. (Esophagus. 
el", Ventral anal arms of Brachiolaria. v. Vibratile cord, anal part. 
el, Dorsal oral arms of Brachiolaria. vl. Vibratile cord, oral part. 
e5, Ventral oral arms of Brachiolaria. : uw. Water-tube, developing the abactinal area. 
e® Odd terminal oral arm of Brachiolaria, w!. Water-tube of Brachiolaria leading to madre- 
Brachiolar arms. poric opening (4), and developing the actinal 
jf’. Branch of water-tube (wv!) leading into f- area. 
jf. Odd brachiolar arm. ww!. Portion of the water-tube of Brachiolaria 
formed by the junction of w and w’. 
1-28. Asteracanthion berylinus. From Alexander Agassiz, Embryology of the Starfish, 1864 (Advance Part 
to 
of Agass. Cont. Nat. Hist. U. S., V.), Pls. I., I. Memoirs Mus. Comp. Zoél., V., No. 1, North American 
Starfishes, 1877, Pls. I., II. 
Figs. 13-18. Scyphistoma stages. Figs. 26-30. Brachina stages. 
Figs. 19-25. Tornaria stages. Figs. 31-34. Brachiolaria stages. 
. Amature egg, surrounded by spermatic particles, soon after 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 outer envelope. 
. The germinative vesicle has disappeared, but the nucleolus remains. 
. The germinative dot is no longer visible ; the yolk has contracted, and is separated by a slight space from the 
outer envelope. The egg at this early stage has all the appearance of having already gone through its seg- 
mentation, the whole yolk being made up of small spherical cells resembling very minute spheres of seg- 
mentation, although the segmentation has not yet commenced. Two hours after fecundation. 
. Shows the first trace of segmentation, consisting in a slight depression on one side of the yolk. 
. The yolk has become flattened on opposite poles ; the ‘t Richtungsblaschen” are visible on one side of the 
yolk. 
. Shows the yolk divided into united ellipsoids : the whole yolk rotates slowly, always in one direction, from 
right to left. The ‘* Richtungsblaschen ” are at one pole of the axis of segmentation. 
The two segments of the yolk have entirely separated. The ‘‘ Richtungsbliischen ” are likewise isolated at 
one pole of the axis of segmentation. 
. The yolk segments are about to separate into four, 
. The yolk, about to divide into eight spheres. 
. The yolk is divided into sixteen spheres. 
. The spheres of the yolk have undergone two additional subdivisions since the preceding stage. 
. The segmentation has been carried on still further ; the spheres of segmentation have become somewhat po- 
lygonal, and form an embryo with a spherical shell consisting of comparatively large cells. 
. 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 (@), and the spheres of segmentation have become somewhat in- 
distinct. 
14. The thickened wall of the flattened pole (a) has formed a slight invagination. 
. The invagination (a) has increased in depth, the spheres of segmentation have entirely disappeared, the invagi- 
nation (a) assumes somewhat the aspect of a rudimentary digestive cavity. Twelve hours after fecundation. 
