128 CRESS WELL SHEARER, WALTER DE MORGAN, H. M. FUCHS. ' 



male gonad when torn exudes a milky fluid, which under the micro- 

 scope appears to seethe with vibratile spermatozoa ; but it is obviously- 

 difficult to compare the relative rapidity of motion in sperms from 

 different individuals. 



III. METHODS. 



A portion of a ripe ovary was placed in a finger bowl containing either 

 " outside water " or " Berkefeld water." * 

 ^ - As soon as the ova floated out, the piece of ovary and any pieces of 

 tissue were removed and a small quantity of a culture of sperm added. 



■ i^^^^'Excess of sperm should be avoided, as it speedily fouls the water, 

 which should be changed if it appears milky. If sperm and ova were 

 ripe, the fertilization membrane was thrown out in a few minutes, and 

 segmentation followed. In about twenty-four hours free-swimming 



^,J>^lastulae appeared. These were removed to jars containing about 

 2000 cc. of Berkefeld or outside water. A couple of small pipettes- 

 full of a culture of the diatom Nitschzia closterium were then added for 

 food, and the blastulae left to develop. 



All the usual precautions for sterilizing jars, pi])ettes, scissors, etc., 

 were rigidly observed, and before being opened the Echini were 

 immersed in tap water to destroy any sperm adhering to the test. In 

 all experiments proper controls were kept, and in any case where these 

 went wrong the entire batch of material was thrown away and the 

 experiment repeated. 



IV. DESCRIPTION OF EXTERNAL CHARACTERS OF 

 NORMAL AND HYBRID LARV^.f 



(a) EARLY DEVELOPMENT OF 



E. ESCULENTVS'i 



The four-armed pluteus stage is reached in about four to six days 

 from date of fertilization. It then has a symmetrical body, well 

 rounded at the posterior pole, with slender arms, longer than the depth 

 of the body. It is slightly pigmented. A few days later (from seven 

 to nine days) the third pair of arms (postero-dorsal) appears, and 

 between the second and third weeks the anterior epaulettes. The 



* In using the term "outside water," we mean water brought into the Laboratory in 

 3-4 gallon glass carboys, collected outside the Plymouth Breakwater in the tidal water 

 of the English Channel, and therefore three or four miles from land. These flasks are 

 "always allowed to stand for four or five days in the Laboratory before being used, thus 

 ensuring the absence of live sperm. By " Berkefeld water " we mean ordinary Laboratory 

 tank-water, which is of considerably lower alkalinity than "outside water," which has 

 been treated with animal charcoal, aerated and filtered through a Berkefeld filter and 

 then stored in sterilized flasks. See Allen and Nelson (1). 



t The nomenclature used in this paper is that of Mortensen (13). 



