H. M. FucHS 245 



cross-fertilized under different conditions. It was thougiit that these 

 experiments were worth recording, and the details, together with the 

 conclusions to be drawn from them, are given in the following sections. 



The first series of observations concerns the comparative rates of 

 development and the condition of the larvae hatched out when different 

 lots of eggs are cross-fertilized at regular intervals after the genital 

 products have been removed from the body of the animal into sea-water. 

 The fertilizations were made in exactly the same way as those described 

 on p. 234 above, and indeed several of the experiments in this section 

 are identical with those recorded in Table V, the subsequent develop- 

 ment of the eggs having been noted. The method it will be recalled, 

 was briefly as follows. P^or each experiment eggs were removed from 

 one individual and sperm from another. At definite intervals (given in 

 the Table below) approximately equal quantities of the eggs were fer- 

 tilized by the addition each time of exactly equal amounts of the dilute 

 sperm-suspension. 



The subsequent observations were as follows : 



III the first place, the percentages of eggs fertilized were counted as 

 usual. 



Secondly, the rates of segmentation of the eggs were compared by 

 noting the length of time after fertilization at which the 4-cell division 

 took place. This stage was fixed upon as it is the easiest to observe 

 rapidly and accurately under a low power of the microscope. Now, 

 although all the eggs in a given lot do not complete the 4-cell division 

 at a given moment, yet in general the majority divide almost simul- 

 taneously, and for this reason the criterion adopted for the comparison 

 of the rates of segmentation was the time at which the first few eggs in 

 a given sample were seen to have completed their second division. This 

 naturally involves a certain latitude of experimental error, and in con- 

 sequence the figures given in the Tables below are not correct to a 

 minute. The extent of this error, however, does not exceed two minutes, 

 and it will be seen that the differences between the times taken to 

 complete the second divisions in the different fertilizations in a given 

 experiment are usually considerably larger than this. 



The rapidities of development of the embryos up to hatching were 

 compared by noting the lengths of time after fertilization at which the 

 first larvae emerged from the eggs in the different dishes. It is usually 

 about half an hour after the first has come out before the majority have 

 emerged from the eggs, and when the larvae are weakly, this time is 

 considerably longer. The comparisons were made by observing when 



