. 19.] ECHINODERMS OF CONNECTICUT. 51 



bottom, the greater part of the water should be poured off and 

 be replaced by clean water. No more eggs should be left in 

 the vessel than will cover the bottom in a single layer. A drop 

 or two of the water in a vessel containing the sperm should 

 now be added to the eggs, and will probably contain a sufficient 

 number of sperm cells to fertilize all the eggs present. 



Within an hour and a half after fertilization, the eggs, which 

 should be examined from time to time under the microscope, 

 will have divided into two cells. The four-, eight-, twelve-, and 

 sixteen-celled stages follow rapidly, and resemble the cleavage 

 stages of the sea-urchin shown in Fig. 15. 



In about twelve hours more, if the temperature is fairly 

 warm, free-swimming ciliated embryos will be found. If a few 

 embryos are kept in plenty of clean, cool sea water, they will 

 live for several days and develop into the so-called brachiolaria 

 stage, as shown in Fig. 4. 



Under the normal conditions of the sea, the free-swimming 

 stages last about three or four weeks, after which such larvae 

 as have survived settle to the bottom and attach themselves to 

 rocks, seaweeds, eelgrass, or other objects, and by a complicated 

 metamorphosis become transformed into tiny starfishes. 



As has been stated above, the majority of individuals of the 

 common starfish mature their genital products in May and June, 

 and discharge them into the water during the latter month. 

 There is, however, great individual variation in this respect, for 

 an examination of a large number of specimens at any season 

 of the year, even in the fall, will show some in which the genital 

 glands are large and contain ripe or nearly ripe sexual products. 

 Furthermore, during the months immediately following the nor- 

 mal spawning season there are some individuals which are filled 

 with mature genital glands. Such individuals apparently repre- 

 sent instances of belated maturity of the sex products, or those 

 which for some reason have failed to discharge their ripened 

 germ cells at the normal time. 



Such individuals, however, probably have little influence in 

 propagating the species ; for, even if their sexual products are 

 discharged into the water, there is very little chance that fertiliza- 

 tion can occur at other times than in the normal spawning season, 

 when the water is swarming with the germ cells of both sexes. 



