472 THE AMERICAN NATURALIST [Vol. LYI 



a female are placed in a finger bowl of clean sea-water. After 

 copulation and egg-laj'ing the animals are removed from the 

 finger bowl. After the jelly has been extruded by the eggs, the 

 supernatant sea-water is poured off, leaving the eggs in the mat 

 of jelly stuck to bottom of the bowl. At first cleavage, which is 

 invariably one hundred per cent., the 'jelly-mass is gently broken 

 up and the eggs efiually distributed among seven to ten finger 

 bowls of clean sea-water. Early the next morning the sea-water 

 is changed. At this time all eggs that possess fewer or more 

 than four oil drops, one in each macromere, are discarded. Only 

 those larvae that possess four oil drops evenly distributed among 

 the four macromeres give rise to normal swimming larv£e. As 

 the trochophores rise to the surface in each dish they are 

 pipetted off. Trochophores that fail to swim at the surface in 

 twenty-four hours lack the viability of those that rise earlier. 



The young larvae are kept in subdued light a few in each dish 

 because they tend to aggregate in such dense masses that many 

 . die off. This tendency to collect in one spot makes it easy to 

 change the water and thus avoid too great rise in temperature, 

 which is fatal to the animals. The larva" will reach the stage of 

 three swimming segments without the addition of food. 



When the segments appear, the larvie must now be watched 

 very carefully in order that food may lie given at the jiroper 

 time. The criterion for the imfial fffding is fhr complete dis- 

 appearance of the oil drops from (he entoderm cells. 



In the eggs of both AV/v/.s jiiid I'/af j/nt ri is i> ;it flic time 



of fertilization a girdle of soin*' cinlurfu to twiity-two oil di-ops 

 in the equatorial zone. Tlie-x' oil (lro!)s in tlif inaturatioii staws 

 following insemination move to the vegetative pole. During 

 cleavage the number of oil drops is reduced to four large glob- 

 ules which normally are distributed to the cells of the gut. Be- 

 ginning with the third or fourth day after laying, the oil in the 

 gut cells of the larva? begins to form an emulsion of smaller and 

 smaller drops. It is thus possible to follow the history of the oil 

 drops very fully in these creatures that make veritable living 

 test tnb(N in a fat-digestion experiment. If food is given the 

 worms l.efoiv tl... oil ha. lu.-n .onipl.tHv um.1. th.-v an- killrd in 

 bu-v niuubor.. (Ml tlir <,tluT han.l. food must not be withheld 



