554 Phylum Echinodermata 



egg laying. In the presence of a greatly diluted suspension of sperm, 

 such as may be assumed to be present under normal conditions in the 

 sea, it is probable that mature eggs only are extruded by females but in 

 aquaria the water becomes clouded with sperm and, thus over-stimulated, 

 the females throw out their entire content of eggs, the immature with the 

 mature. 



A female while extruding eggs either stands upright, supported by the 

 side of the aquarium, or with her body held horizontal to and high above 

 the bottom by her strongly arched arms. While the body is thus elevated 

 the eggs pass from the distal openings of the genital bursae into the 

 water and slowly rise to the surface, having the appearance of ascending 

 streams of minute bubbles of air. The eggs may now be taken with a 

 pipette and placed in smaller containers of fresh seawater where develop- 

 ment will take place and continue to a stage in which the adult structure 

 is attained, provided daily transfers to fresh containers and seawater are 

 made. The eggs are large, about 0.3 mm. in diameter, and contain an 

 amount of yolk sufficient for the energy requirements of complete de- 

 velopment. The final stages are reached earlier however if a supply 

 of diatoms or diatom-bearing sand is added to the water [see p. 31], 

 food being thus provided for the developing stars. 



Bibliography 



Grave, Caswell. 1900. Ophhira brevispina. Mem. Nat. Acad. Set. 

 1916. Ophiura brevispina. II. J. Morph. 27: 



Class Echinoidea, Order centrechinoida 



ARBACIA PUNCTULATA 



Henry J. Fry, Cornell University Medical College 



AT the Woods Hole Marine Biological Laboratory the eggs of the sea 

 l\ urchin are used for experimental purposes probably to a greater 

 extent than any other material. During the summer of 1935, 62,000 

 Arbacia were supplied to investigators. There are good reasons for this 

 popularity: The season usually extends from the middle of June to the 

 middle of September; one female yields about 0.5 cc. of "dry eggs"; to 

 an unusual degree these eggs withstand the abnormal conditions of vari- 

 ous experimental procedures and yet develop like the controls (for 

 example, fertilized eggs placed in a Warburg apparatus will cleave 

 normally in spite of great crowding, continuous shaking, and no change 

 of water for several hours) ; on the warmest days, room temperatures do 

 not interfere with normal development; and eggs which have stood for 



