DKKKCTIVK PLUTEUS LARV/E. 



the third clay the skeletal material had extended over to the other 

 side and a second postoral arm had appeared. The oral part of 



A 



FIG. 5. Successive drawings of a pluteus larva developed from a single blasto- 

 mere of the two cell stage of Arbacia. The skeleton was first one -sided, but later 

 all except the oral portion was regenerated, A two days old, B three days old, 

 C five days old. An aboral and a side view are shown in each case. Measured 

 and drawn free hand to the same scale from the living specimen. 



the skeleton was defective. By the fourth day a skeleton with 

 all parts represented, but abnormally massive, and of rather less 

 than the usual size was formed. 



CLEAVAGE OF ISOLATED BLASTOMERES. 



The work of Driesch, of Morgan, and others indicated that the 

 cleavage of isolated blastomeres of Echinus and Sphar -echinus was 

 that of half embryos. Each blastomere divided into four equal 

 cells, and at what corresponds with the fourth cleavage of the 

 original egg gave rise to two micromeres, two macromeres and 

 tour mesomeres, just as they would have done in the normal egg. 

 They later formed an open blastula with halt the normal number 

 of cells, which eventually rounded up. Since in the cleavage of 

 Echinarachnius there are no clearly distinct micromeres, it is 

 impossible to compare its cleavage with that <>t the others. The 

 cleavage ot the one half blastomeres ot Arlxn in was followed, but 



