EMBKYOLOGY OF ASTEEIAS. 



183 



occurs during the spring months, at which period the ovaria of the 

 females are found distended with eggs, wherein the vesicles of Purkinje 

 and of Wagner are distinctly recognizable. These ova are found in the 

 ovaria in different stages of development, and are laid in successive 

 batches at different intervals. 



(481.) The newly-laid ova consist of a chorion enclosing the vitellus 

 and a small quantity of albumen ; but the vitellus soon undergoes the 

 usual process of segmentation, whereby it is broken up into a granular 

 mass (fig. 91, 4, 5, 6, 7, 8). When first deposited, the ova of the Star- 

 fishes are not at once abandoned by the parent animals, but are retained 



Fig. 91. 



Development of Star-fish. 1. EcUnaster sanguinolentm seen from below. 2. The same in pro- 

 file : a, madreporic plate. 3. Ovarian receptacle containing ova in different states of advancement. 

 4, 5, 6, 7, 8. Ova exhibiting the progressive segmentation of the yelk. 9. Embryo on its first 

 escape from the egg. 10, 11, 12. Further progress of embryo : a a, club-shaped processes ; b, cen- 

 tral protuberance. 13, 14, 15, 16, show the gradual development of the ambulatory suckers and 

 the assumption of the radiate form. 



in a kind of cavity formed by incurving the body and rays of the mother 

 until they form a sort of chamber, beneath which the eggs are protected 

 during the earlier part of their development (fig. 91, 2). The vitellus 

 of the ovum is entirely employed in the construction of the foetus, which 

 latter, at the moment of its escape from the egg, is of an ovoid or sub- 

 spherical shape (fig. 91, 3), completely unprovided with external mem- 

 bers, but enabled to swim vivaciously about in the surrounding water 

 by means of the cilia with which its body is profusely covered, giving 

 it exactly the appearance of an infusorial animalcule ; indeed, this may 

 be called the first, or infusorial condition of the young Asterias. 



(482.) After the lapse of a few days certain appendages begin to 



