Experiments on the Early Development of the Amphibian Embryo etc. 621 



yolk cells, except to darken their borders, and to color the cells 

 along both sides of the archenteric cleft. Jordan has suggested / 

 that the relative lack of pigment in certain cells is due to a »more 

 sluggish metabolism attendant upon less rapid cell division « (30). 



We do know that where cell division is especially rapid, as in 

 the vertebral somites and the ventricles of the brain, there is always 

 an accumulation of pigment. The same thing exists in these Am- 

 blystoma embryos, but other conditions indicate quite conclusively 

 that such an accumulation cannot be due merely to absolute activity, 

 nor can the lack of it depend on sluggish division. The salt solution 

 retards development and cell division is more sluggish than in nor- 

 mal embryos. This is especially true in the yolk cells. But in these 

 embryos and more noticeably in those reared in Ringer solution, 

 where the division is even more sluggish, nearly every yolk cell 

 contains a large amount of pigment gathered around it's nucleus. 

 In the external cells it appears at the surface. We have here, then, 

 an increase in pigment combined with a decrease in activity. 



The cells within the blastopore crescent are first very large and 

 entirely free from pigment. But they soon divide and each of the 

 resultant cells acquires a small amount of pigment near its center 

 (fig. 2). In subsequent division the pigment is also divided and 

 appears at the center of the resultant cells. It gradually increases 

 in amount until at the formation of the neural folds it appears at 

 the center of all the cells (figs. 3 and 4). At the closing of the 

 folds (fig. 5) the pigment spots cover a large portion of the cell, 

 and still later in development (fig. 6) they have increased sufficiently 

 to color the whole surface of the yolk nearly as dark as the sur- 

 rounding ectoderm. The cells themselves, howeyer, remain very 

 large compared with the ectoderm cells. There seems to be, there- 

 fore, a well defined growth of pigment in situ, and in all subsequent 

 development the surface of the yolk is approximately the same color 

 as the ectoderm, but traces of the peculiar mottled appearance remain 

 for some time (fig. G). In the normal embryo such yolk cells are 

 not very active and they are still less so here. But there is, in all 

 probability, a greater difference in the relative activities of nucleus 

 and protoplasm in these eggs, and hence, compared with its sur- 

 roundings, the nucleus may be considered as quite active. It is 

 probably this relative activity of the nucleus, rather than any absolute 

 activity, which it certainly does not possess, that accounts for the 

 accumulation of pigment around it. 



