EFFECT OF EXTRACTS UPON DEVELOPING EGGS. 77 



actual minutes after fertilization, the late gastrula becomes a 

 pluteus for instance, by careful comparison of the culture with 

 the control one can observe any differences in their stages of 

 development. 



Two methods of comparing rates of cleavage were employed : 

 the time elapsing between insemination and the first 2-, 4- or 8-cell 

 stage, or the period between insemination and the time the culture 

 showed a 50 per cent, development of a given stage. For obvious 

 reasons the former method was used for the most part, only the 

 time required for making a single observation elapsing between 

 the readings, and, in order that there might be no errors introduced 

 merely by the arrangement of the culture dishes, the order of 

 readings was frequently reversed. Equal quantities of eggs from 

 the same suspension were used for each culture to insure an equal 

 opportunity for oxidation and presumably then an equal concen- 

 tration of CO 2 in the water. The cultures were kept in flat-bot- 

 tomed finger bowls and were stirred frequently. During the 

 season the temperature of the room varied from 21 to 23.5 C. ; 

 the variation in the temperature of the sea water in the circula- 

 tion as recorded .each day was found to be from 18.1 C. to 20.5 

 C. The pH of the sea water was tested each day and was found 

 to be very constant 8.2 or at a few times 8.0 when thymol blue 

 was the indicator used. Whenever the quantity of extract was 

 sufficient to admit of its being tested, the pH was found to be 8.0 

 or slightly more acid. It did not appear that there was a charac- 

 teristic difference between the degrees of acidity noted in the 

 extracts made of larva? in different stages of development. 



A new supply of animals was brought into the laboratory each 

 day. These were kept in an aquarium placed on a cement water 

 table with a stream of fresh water running into the aquarium 

 constantly. 



The basis of comparison between a normal and a retarded cul- 

 ture in all experiments was (i) rate of development, (2) size, 

 (3) vigor (rate of movement), (4) longevity. 



Sample tables are not given for the experiments on the starfish 

 because the rates obtained for both the earlier and late stages were 

 so varied that no definite conclusions can be fairly drawn from 

 them. These variations were doubtless to be explained in part 



