CHAPTER 8 



Species of Echinoidea near Woods Hole, Mass, 

 and Arbacia lixula (pustulosa) 



There are three species of Echinoidea which occur in the vicinity of 

 Woods Hole. Arbacia punctulata is, or has been, the most common species 

 and the one most used for experimental work as the eggs are ripe most 

 all summer. 



The "green" urchin, Strongylocentrotus drobachiensis, has been taken in 

 deep water in Vineyard Sound, off Gay Head, Nantucket, and many 

 places off Cape Cod, Barnstable, Chatham, Provincetown, and off 

 George's Bank 75 miles east of Cape Cod. It is a northern species, 

 being circumpolar in the Arctic, and occurs in the north on both the 

 Atlantic and Pacific coasts of North America, on the east coast as far 

 south as New Jersey. It breeds, however, in the early spring, and is 

 spent and of no use for experimental work at Woods Hole during the 

 summer. It occurs in great abundance at the Mt. Desert Laboratory 

 at Salisbury Cove, Maine, and is ripe there also in the spring, March 

 and April. It is full of (mostly) unripe eggs in January and February, 

 full of ripe eggs in March and April, and is spent by May 15, having 

 no ripe eggs in the summer and fall, though some males have ripe 

 sperm. The animals were shipped to me to Princeton for several years 

 (1936-1942) in the spring, for work on hybridization (E. B. Harvey, 

 1942). The eggs were fertilized and developed normally to plutei, 

 provided they were kept in the cold, at about 10° C. 



The egg of Strongylocentrotus drobachiensis is 160 [x in diameter, is un- 

 pigmented and has a jelly coat about 96 jx thick. After fertilization, 

 there is a large perivitelline space of about 30 [x; the hyaline layer is 

 well formed after an hour (at 10° C.) when it is about 3 \i thick. First 

 cleavage takes place in 2-3 hours at 10° C. With centrifugal force, 

 the unfertilized mature egg stratifies with oil at the centripetal pole, 

 then yolk granules, clear layer, and mitochondria at the centrifugal 

 pole; the nucleus (14 (j. in diameter) lies under the oil cap. There is 

 also, sometimes, a clear layer under the oil instead of, or in addition to, 

 the clear layer beneath the yolk. The egg breaks with a centrifugal force of 

 12,000 X g for twelve minutes, just above the clear layer under the 



