66 



JOHN \V. MACARTHUR. 



development is less general and more localized and differential. 

 The eggs produce practically 100 per cent, modified blastuhr, 

 which are smaller and thicker walled than their controls; the 

 ectoderm does not thin out for some time even though the blas- 

 toccel is almost invariably solidly filled with mesenchyme. Such 

 overproduction of mesenchyme seems to lead to a basal extrusion 

 and loss of excess proliferated primary and secondary mesenchy- 

 mal cells from the basal pole of the blastula, which is commonly 



s 



FIG. 3. Commonest types of sand-dollar gastruki* and larvae from lithium rul- 

 Uircs. Show the wide range of lithium alterations. (See also other cxumpl'"- in tin- 

 originals of Fig. 12). a-i, from M/i6o and j-x from M/aoo LiCl, return* .1 at about 

 2 days to sea water. Mesenchyme omitted in most. 



