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and start development. Second, How far will development proceed in the 
various combinations, and in what respects are the processes normal and 
abnormal? 
In every combination effected the earlier phases of cleavage are passed 
through in a perfectly normal manner. The same is true of the later stages 
of cleavage excepting the rate of development. This will be further consid- 
ered below. From the late cleavage on, the history of the different hybrids 
becomes much more varied. In those hybrids resulting from species nearly 
related—belonging to the same genus or to closely allied genera—most of 
the embryos may complete their development to the point of hatching, or 
beyond. Even among these, however, a number variable but much greater 
than in normal embryos, may show abnormalities along the course of their 
development, such as occur more abundantly in the hybrids between more 
distantly related forms. Hybrids between species more distantly related 
than above indicated, so far as my experiments go, never complete their de- 
velopment to the point of hatching. The stage to which they will go depends 
again upon the nearness of their relationship. In the more successful of 
such distant crosses Fundulus-Menidia hybrids, many of the embryos may 
go far enough to form fairly well developed eyes, ear vesicles, tail, muscles, 
central neryous system, heart, color patern, fins, etc., but many of these 
structures in the later stages are variously abnormal. <A large proportion of 
all the embryos, however, fail to reach such advanced stage. From these 
hybrids we have almost every condition to such as obtains in the hybrids 
between Fundulus heteroclitus x Tautogolabrus adspersus, where none of 
the embryos go much beyond the closure of the blastopore, and where it is 
not possible to speak of the formation of organs. ‘The more characteristic 
and striking abnormalities appearing beyond the cleavage stage in these 
various hybrids muy be briefiy considered. 
In the last stages of cleavage and during the earlier phases of germ 
ring formation it is usually not possible to distinguish the hybrids from the 
yormals excepting in the stage of advancement. In some combinations, such 
as Fundulus heteroclitus x Tautogolabrus adspersus, etc., one can very com- 
monly see the formation of a rather large clear area under the blastodise 
Which is filled with a clear fluid. I have followed such eg xs and they do 
not bring their development to as advanced a stage as those eggs of the 
same lots that do not show this abnormality. ‘They may form a very good 
embryonic ring and shield and may overlap the yolk for a third of the way 
