By assuming the weak lens focal length expression to be the first term in a power series a simple and accurate formula can be derived for the objective focal length of a magnetic electron lens.

The fissure lens group, in en echelon arrangement, is ease to be developed and conserved in the weak strain domain of tectonic lens.

Results indicate: (1) At stage 19 the lens plate separated from the presumptive corneal epithelium and weak fluorescence appeared in its basal cells.