500 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In the 32-cell stage Barrois found the embryo to consist of eight larger cells 

 about the vegetative pole and 24 smaller cells, as Seeliger describes it in A. adriatica. 

 Bury makes no mention of a difference in the size of the cells. 



Bury notes that the first cleavage furrow makes its appearance soon after 

 midday, probably at least three hours after fertilization. Subsequent divisions 

 occur at intervals of about an hour. 



The formation of the blastosphere is not completed until fully 12 hours after 

 the first cleavage. 



Gastrulation begins about 20 hours after fertilization (17 hours after the 

 appearance of the first cleavage furrow) ; the blastopore attains its greatest 

 dimensions about the middle of the second day, and closes completely about 40 

 hours after fertilization. 



Apparently no cilia are developed until after the closure of the blastopore. 

 Early on the third day the embryo is covered with a uniform coating of cilia. 



By the fourth day the embryo has nearly lost its pink color and has become 

 white or faintly yellow. It has also become somewhat more elongated and, 

 though still ciliated all over, the body surface has begun to show distinct ciliated 

 bands. 



By the fifth day the embryo has acquired almost all the external features of 

 the completely developed free-swimming larva. The general color is usually 

 yellowish brown, but this color is confined to the nonciliated portions, so that the 

 ciliated bands and the two ventral depressions (also ciliated), which are white, 

 are very clearly defined, their degree of distinctness varying with the amount 

 of pigment developed elsewhere. The greatly elongated cilia of the anterior tuft 

 also spring from a nonpigmented area. 



Besides the yellowish brown cells the nonciliated areas also contain a number 

 of the "yellow cells." By reflected light, if much pigment is developed elsewhere, 

 these cells are very inconspicuous; but when teased out and viewed by transmitted 

 light they appear bright green, while the other ectoderm cells, in which but little 

 pigment is present, appear colorless. 



Bury found that in almost all cases the ciliated tuft is directed forward; 

 indeed, out of hundreds of larvae examined he never saw one swim in the opposite 

 direction, except when it encountered an obstacle, in which case it would reverse 

 its motion for a short distance always, however, resuming before long its original 

 course. The fact that when at rest the cilia all curve toward the anterior end is 

 probably connected with this mode of swimming. 



The first rudiments of the skeleton make their appearance early on the 

 sixth day. Bury could discover no indication that the terminal stem plate, basals, 

 and orals are developed in any particular order, though he considers it probable 

 that all of them are developed before the columnals. 



Early on the seventh day the infrabasals make their appearance at the 

 posterior end of the series of columnals. They are three (rarely four or five) in 

 number, and in form resemble small basals, though they are developed at a much 

 deeper level than these plates, and are usually nearer the posterior end of the 



