No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 269 



a few desrrees between the first and the last of the breedina" 

 season appreciably increases the rate of development. If arti- 

 ficial heat is applied, even a sudden rise of many degrees does 

 not have the slightest effect on the manner of development, but 

 greatly increases the rapidity. Of several hundred eggs ferti- 

 lized at 8 A.M. on May i, the larger number were in the 2-cell 

 stage at 1 1.30 a.m., though some of them were dividing into four. 

 The eggs were then separated into two dishes. The tempera- 

 ture in dish A was kept at that of the sea, 8° C. The other 

 dish, B, was placed in a warm bath, and at 1 1 .40 the tempera- 

 ture had risen to 35° C. It rapidly fell to 21°, and there re- 

 mained. In dish A the development continued very slowly. 

 In dish B, where the temperature was raised 17° in ten minutes, 

 the eggs took a sudden start and developed twice as quickly 

 as those in the cold water. 



Dish A, 8° C. Dish B, 21° C. 



1 1 .40 A.M. 2- 4 cells, , . 2- 4 cells. 



12.07 P-M. 2- 4 



1.05 " 4- 8 



3.3s « 8-16 



4.00 " 16 -l- 

 4.10 " 



4- 8 " 

 16-32 " 

 32-64 " 



64 -[- " (swimming), 

 swimming rapidly. 



The reaction of larvae to light varies with age. When they 

 first begin to swim, they are positively heliotropic, but later, 

 when they are about fourteen hours old (text Fig. XX), most of 

 them are negatively heliotropic. From twenty to about forty 

 hours after fertilization they are apparently indifferent to light, 

 and are scattered evenly throughout the water. At about forty- 

 four hours they exhibit a very marked positive heliotropism, 

 which continues for several days at least. 



Larvae three days old which were strongly heliotropic, crowd- 

 ing to the sides of the dish nearest the window, could not be 

 induced to abandon this position by direct heat rays acting 

 opposite to, at right angles to, or coincident with, the 

 light rays. 



