383 
sperm species is more rapid or slower than that of the egg species. Thus. 
reference to Table 3, where Fundulus heteroclitus was the egg species and 
Tautogolabrus adspersus was the male species, the rhythm of cleavage fol- 
lows exactly that of Fundulus, although that of Tautogolabrus is very much 
faster. The reciprocal shown in Table 4 shows that the rate again is that 
of the egg species—Tautogolabrus. This is true all the way through, but 
attention is called to the hybrids with Opsanus tau, where the cleavage 
rhythm is relatively so extremely slow. (Page 375. These facts are in 
accord with many observations made by others, especially Driesch (798) 
on Echinoderms. Newman (‘O8, °10) obtained the same results in his 
Fundulus heteroclitus 
Fundulus majalis hybrids. Fischel ((06), on the 
contrary, maintains that the influence of the sperm in some of the Echino- 
derm hybrids, makes itself felt even in the first cleavage. It is important 
to note, however, that such influence as he can detect is always to slow 
the development. This is what I find everywhere, as will appear further 
on, but I have not been able to detect it during the early cleavage stages. 
This slowing of the developmental processes is to be looked upon as patho- 
logical, a sort of incompatibility of the two germinal substances in such 
cases as it occurs. If it is permissible, as some authors do, to speak of the 
rhythm of cleavage as a character of the organism, then all my experiments 
most clearly show tbat the rate of earlier cleavage of the embryo is unin- 
fluenced by the sperm, and may be regarded as wholly determined by 
the egg. 
In later cleavage and all subsequent stages, the influence of the 
strange sperm becomes apparent in all the cases that I have carefully 
watched. It should be said here that hybrids between the nearly related 
species were not studied in this particular, but only those between the more 
distant forms. The influence of the strange sperm was in every case to 
retard development, usually to a marked degree, regardless of whether 
the developmental processes in the sperm species was much more rapid or 
slower than in the egg species. Thus Tautogolabrus adspersus takes only 
from twenty-four to thirty-six hours to hatch, while Fundulus heteroclitus 
takes from ten days to fourteen days, the hybrids, using Fundulus as the 
egg species, are slower in their development than Fundulus itself. The 
tendency, then, among fish hybrids obtained by combining distantly related 
species, is to develop slower after their earlier cleavage stages, than the 
ege species. It is, therefore, interesting to note Newman’s result where 
