622 



Chas. B. Wilson 



The irregularity of the border between the pigmented and the 

 light area is very noticeable during gastrulation. This is well shown 

 in fig. 1. At some points the pigment extends far up toward the 

 blastopore crescent in loose irregular streaks and blotches, while 

 elsewhere it has been advanced but little beyond the equator. The 

 cells in the dark blotches are of the same size as those in the light 

 areas at the same level, and are much larger than the ectodermal 

 cells. This irregularity cannot be explained, therefore, by a differ- 

 ence in the down growth of the pigmented cells of the upper 

 hemisphere. It persists even after the formation of the blasto- 

 pore ring and must be due to the inequality of the yolk cells 

 themselves, and the consequent difference in the influence of the 

 salt upon them. 



A study of sections shows that no ectoderm is formed over the 

 yolk plug, but that this darkening is due to the formation of pigment 

 in situ (figs. 12 and 13). 



TsuDA (39) found in Rana temporaria that »the growth of pig- 

 ment does not correspond in any way to the growth of new ectoderm 

 cells, but .... varies in various lots of eggs of the same stage 

 procured at different times «. That is, eggs from different females 

 vary considerably in the amount of pigment they contain at any 

 given stage in development. 



But she further states that »all the eggs of the same stage from 

 one cluster are alike in quantity of pigment«. In these eggs not 

 even this was true, but different eggs from the same cluster varied 

 considerably in the amount and distribution of pigment. 



TsuDA concludes that the amount of pigment varies with the 

 conditions previous to the beginning of segmentation. It would seem 

 fair to add that conditions may also arise during segmentation 

 which will modify the pigment considerably: or even cause it to 

 appear abundantly in places where it is not usually found at all. 



The Fate of the Blastopore. 



Concerning this much discussed question the embryos developed 

 in salt solution present many interesting details. 



In the first place the real abnormality in such development 

 lies in it's retardation. These Amblystoma eggs produced in twenty 

 days embryos which in outward appearance were exactly like others 

 reared normally in a little more than half the time (fig. 8). And a 

 study of sections did not reveal any essential difference. 



