390 



comes very large and the heart is commonly drawn out to a filamentous 

 form. This continues to beat until the death of the embryo, but does not 

 handle any blood. The eyes do not attain their full size, and may be poorly 

 pigmented. They often are abnormally set so that they occupy the fore- 

 part of the head. This may fuse into a median single eye or may be pres- 

 ent on one side only. The ear vesicles often become large and inflated, giv- 

 ing rise to a large rounded prominence on each side. The pigment cells 

 are very finely developed, show a tendency to a pattern and bilateral sym- 

 metry, but there is a lack of uniformity in this in the different embryos. 

 The embryos are shortened and may develop abnormally large pectoral 

 fins. It is not necessary to give more than a general description at this 

 place. 



Even within this group it is very easy to distinguish between the 

 hybrids in which the Menidia notata is used as the male from those in 

 which the Menidia gracilis is the male. The development of the former is 

 more successful in those that pass the blastopore closure stage, although 

 my experiments show that the mortality is greater at this point. The pig- 

 mentation is better developed and the various organs above mentioned 

 are laid down much more normally. As a consequence fewer and less pro- 

 nounced abnormalities occur. In the Fundulus-Menidia gracilis cross it is 

 not uncommon to have only one eye formed. This may be lateral or may 

 be median. The eyes are commonly set much further anterior so as to 

 occupy the front of the head than in the nearly related cross. 



In the crosses of group 3 we obtain quite a difiierent series of 

 hybrid embryos. None of these will develop as far along as those in 

 group 2. There is the usual large mortality preceding and at the blas- 

 topore closure stage. The more successful embryos are much shorter, the 

 pigmentation is much less perfect, the black usually predominates, the eyes 

 are never normal, and often altogether wanting, and the life of the embryo 

 is shorter. The heart and pericardial cavity is much the same as. in the 

 Menidia hybrids, although I have seen no attempt to develop vessels on the 

 yolk. Their embryos show in every way that the developmental processes 

 have deteriorated much earlier than in the Menidia crosses. 



When we come to group 4 we have a still more pronounced abortion 

 of the developmental processes. Many of the embryos close the blastopore 

 after a fashion, but the embryo is always much shortened, usually being a 

 mere streak of protoplasm. These embryos do not lengthen to form a 



