22 



EMBRYOLOGICAL MOXOGEAPHS. 



PLATE VII. 



Dsvelopment of Asteroidea, continued. Figures from Alexander Agassiz. 



a. Anus. 



h. Dorsal or wnter pore, inadrcporic opening. 



c. Alimentary canal. 



d. Digestive cavity. 



f'. Median anal arms of Bracliiolaria. 



c". Dorsal anal arms of Brachiolaria. 



c'". Ventral anal arms of Brachiolaria. 



c"". Dorsal oral arms of Brachiolaria. 



c'. Ventral oval arms of Brachiolaria. 



c^. Odd terminal oral arm of Biachiolaria. 



/. Brachiolar arms. 



f. Branch of water-tube {lo u-') leading into/. 



f. Odd brachiolar arm. 



f". 



Surface warts at the base of the odd brachiolar 



arm (/"). 

 Mouth. 



Pistol-shaped oral pouch of oesophagus. 

 Anal pouch of oesophagus. 

 Q?sopliagus. 



Vibratile cord, anal part. 

 Vibratile cord, oral part. 

 Water-tube, developing the abactinal area. 

 Water-tube of Brachiolai'ia leading to madre- 



poric opening (J), and developing the actinal 



area. 

 wv}l. Portion of the water-tube of Brachiolaria 



formed by tlie junction of iv and v)'. 



m. 



m', 



V'. 



V). 



vii 



2S. Asfi'rnc'inthion bcryUnun. From Alexander Agassiz, Embryology of the Starfish, 1864 (.Advance Part 

 of Agass. Cont. Nat. Hist. U. S., V.), Pis. I., II. Memoirs Mus. Comp. Zool., V., No. 1, North American 

 Starfishes, 1877, Pis. I., II. 



Figs. 13-18. Scyphistoma stages. Figs. 26-30. Brachina stages. 



Figs. 19-25. Tornaria stages. Figs. 31-34. Brachiolaria stages. 



A mature egg, surrounded by spermatic particles, soon after artificial fecundation. The egg lias assumed 

 a spherical shape, and contains the germinative vesicle and dot. There is no trace of any interval between 

 the yolk anil 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 " Richtungsbliischen " 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 " Kichtungsblascheu " are likewise i.solated 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 s]ih(^res 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 bec(une 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 

 liiMome very nuicli thickeneil at one pole (<(), and the spheres of segmentation have become somewhat in- 

 distinct. 



Tlie thickened wall of the flattened pole (n) has formed a slight invagination. 



The invagination (a) has increa.sed in depth, the .spheres of segnumtation have entirely disappeared, the invagi- 

 nation (re) assumes somewhat the aspect of a rudimentary digestive cavity. Twelve houis after fecundation. 



