AXIAL SUSCEPTIBILITY GRADIENTS. 399 



and that in cases like that in Fig. n, where a portion of the 

 cytoplasm is cut off, it is the apical region, which is so much 

 injured that it cannot give rise to a cleavage furrow. 



With the same methods of treatment a general gradation in 

 the size of the blastomeres was observed in 3040 per cent, of the 

 eggs in stages from 32 cells onward, though it became less marked 

 in later stages as recovery proceeded. In these cases also it is 

 probable that the region of most rapid cleavage and therefore of 

 the smallest blastomeres is the basal region and the region where 

 cleavage is most inhibited and therefore the blastomeres are 

 largest is the apical region. 



The small size and protoplasmic character of the micromeres 

 at the basal pole suggests the possibility that their metabolic rate 

 may be higher than that of the cells about them, but I have riot 

 been able to discover that their susceptibility is greater than 

 that of the adjoining cells, and the indirect evidence shows that 

 the mesenchyme cells, which are believed to be at least in part 

 descendants of the micromeres are among the least susceptible 

 if not the least susceptible of all the cells. 



These observations on the earlier developmental stages are not 

 conclusive. Taken by themselves they are of little value, but 

 considered in the light of the indirect evidence and of the more 

 definite results obtained in later stages they possess a certain 

 significance as contributory evidence. My observations on these 

 stages were only incidental to other work and only KCN was 

 used as reagent. 



THE BLASTULA AND GASTRULA. 



As soon as the blastula begins to elongate the direction of 

 movement, and in more advanced stages the greater thickness 

 of the cellular wall in the basal region make it possible to dis- 

 tinguish apical and basal ends without difficulty. In these 

 stages death begins at the apical end and proceeds basally (Figs. 

 12-15), advancing in many cases somewhat more rapidly down 

 one side (Figs. 13, 14), probably that side which later becomes 

 the anterior end of the pluteus. The progress of death in the 

 basal direction is very regular, though a few cells here and there 

 may swell and push out of the body-wall earlier than others 



