1 64 LOEB. 



twenty-four hours after the impregnation. A small percentage 

 of these embryos formed a heart, a circulatory system, arteries, 

 veins, capillaries and blood corpuscles. But I was never able 

 to find any sign of a heart-beat or circulation, although 1 

 watched the eggs continually. Nevertheless, these embryos 

 kept on developing until the fourth day after the impregnation. 

 A great number of chromatophores were formed, but not the 

 slightest relation between these and the blood vessels was 

 visible. In normal sea water by this time the first traces of 

 migration were visible. It is my intention to give these exper- 

 iments more detailed in another paper. As far as the causes 

 of the orientation and migration of the chromatophores are 

 concerned, they rather favor the idea that the stimulus for this 

 orientation and migration is to be sought in the circulating 

 blood. 



5 . From the above we conclude that the typical coloration of 

 the yolk sac of the Fimdiiliis embryo is due to a specific, probably, 

 chemical irritability of the chromatophores which are forced by 

 this irritability to migrate to the blood vessels and gather on 

 their sniface. If that be true we may expect that this same 

 irritability also plays some role in the coloration of the embryo 

 itself. I intend to study this question more in detail next 

 summer, and will give here only a few facts which possibly 

 answer the question affirmatively. It seems as if there were 

 formed in the embryo two rows of pigment on either side 

 corresponding to the cardinal veins. It may be, moreover, that 

 the two dark spots which are formed, one near the tail, the 

 other nearer the head of the embryo, correspond to the places 

 where the vitelline arteries leave the body of the fish, and 

 where indeed plenty of chromatophores are found. But be this 

 as it may, it is important to remember that our explanation of 

 coloration only can be applied in cases in which coloration is 

 determined by chromatophores. And even in such cases we 

 must not forget that chemical irritability is not necessarily the 

 only kind of irritability of chromatophores. In certain animals 

 and organs for instance, they are irritable by light. It is 

 possible that in cases where blood vessels determine the orien- 

 tation of pigment cells this only takes place as long as the walls 

 of the vessels are sufficiently thin, as in the embryo. 



