DEVELOPMENT OF SPELERPES BILINEATUS 229 



When the observations were made it was supposed that the 

 shape of the blastopore would enable one to determine the com- 

 parative age of the egg, but this method has little value. The 

 only sure way is to isolate individual eggs and to observe the first 

 appearance of the blastopore in each egg. 



All the eggs described were marked and the movements of 

 the blastopore studied by inverting the eggs at intervals. The 

 secret of using two slides to invert the egg was not known when 

 these observations were made. The lack of this knowledge pre- 

 vented as prolonged studies on each egg as was desirable, for, 

 in general, much handling and especially inversion of the egg 

 is injurious, although apparently it does not affect the movements 

 of the cells. Moreover, the eggs were usually marked with auran- 

 tia, which usually fades out in about twenty-four hours. 



Series B. No. 4, figs. 64-68 ;i- marked at 7:55 a.m. (fig. 64). 

 There was a large spot immediately above the blastopore and 

 a small one about 45° ventral to it. At 11:50 a.m., (fig. 65), the 

 blastopore had extended to a semicircle. The mark above the 

 lip had elongated and apparently narrowed, although this last 

 change might have been due merely to a change in the angle 

 of vision. There was a new mark at the right, extending ventrally 

 from the corner of the blastopore. It arose doubtless from stain 

 left in the jelly. About three hours later, viz., 3:10 p.m. (fig. 

 66), the blastopore had retracted into a small arc, which appar- 

 ently is derived entirely from the previous right half, just as 

 though the left half had been erased from fig. 65. Each of the 

 larger marks lay anteriorly above its respective corner of the 

 blastopore. The small ventral mark had continued to approach 

 the blastopore and in the next drawing, at 6:50 p.m. (fig. 67), 

 lay close to it. The other marks did not change to any great 

 extent except that the right one narrowed. The last drawing 

 of this egg (fig. 68) was made the next morning. The blasto- 

 pore is now bent dorsally at one corner. The ventral mark 

 had disappeared, presumably into the blastopore. Both dorsal 



1- The blastopore is hard to see in the living egg, owing to its lack of pigment. 

 Some of the minor differences recorded in the extent of the blastopore are due to 

 this fact. 



