Our study of the effect of the lead ion on the development of the chick embryo has brought out the following facts:
1. Gross growth is retarded.
2. Somite growth is retarded to a degree greater than that exhibited by body length and width.
3. The head and optic anlagen are regions of particular sensitivity. Their differential development is markedly inhibited.
From the purely biological point of view these results are in line with the findings of Child (10) and his school as to the sensitivity of the head end of rapidly growing organisms to harmful influences, and with those of Stockard (11) as to the peculiar sensitivity of the optic anlagen.
It is almost too well known to need repetition that the head region and the somites of embryos are specific areas of intense growth by increase in cell number. Therefore, turning from the general to the particular, the differential retardation of these regions which is caused by lead, is evidence justifying the conclusion that it is areas of rapid growth by cell proliferation which are selectively inhibited by this metallic ion.