574 HAROLD HEATH, 



easy interpretation. Under certain circumstances the cell boundaries 

 appear clearer when the egg is some shade of brown. In such cases 

 treatment with hydrogen peroxide brings about the desired result. 



For study the eggs are brought into a watch glass, and are 

 prevented from rolling by placing in with them some finely cut camel's 

 hair. In this way the eggs remain in position, and free hand drawings 

 are readily made. 



Picro-sulphuric and picro-acetic also gave excellent results, espe- 

 cially the latter, but eggs must be kept in these fluids for five or 

 six hours, otherwise the eggs are so broken or distorted by the 

 process of teasing out, that their study is unprofitable; but after a 

 period of six hours the albumen assumes a condition somewhat similar 

 to that produced by Flemming's fluid, and the eggs may then be rid 

 of their membranes. This latter process is rather slow after these 

 fluids, but it may be done at any time within tw(^ weeks if the eggs 

 are kept in a mixture of equal parts of 95°/o alcohol and 4^/o 

 formalin. 



For surface views Delafield's haematoxylin with a light secondary 

 stain of eosin gave the best results. Sections were made in paraffine 

 and stained in Delafield or Biondi-Ehrlich. In running whole em- 

 bryos through a clearing agent, care must be exercised not to put 

 them at once into one of high difi"usibility, e. g. xylol, which reagent 

 splits the cells asunder. Cedar oil answered best in all cases. In 

 surface views the nuclei stained with Delafield's haematoxylin are 

 quite distinct, but the eggs are filled with a finely granular yolk that 

 often gives an indistinctness to the cell boundaries in later stages, 

 and it is a tedious process to determine them ; but by combining 

 the study of eggs killed in Flemming's fluid, where the boundaries 

 are clear, with a study of stained preparations a perfect interpretation 

 is possible. 



4. General Sketch of Development. 



The cleavage of Ischnochiton is total, nearly equal, and the early 

 cleavages conform to the radial type. The relation of the first two 

 cleavage planes to the axes of the future embryo could not be 

 determined, since definite landmarks are lacking until the 28 cell 

 stage. Three quartettes of ectomeres are cut ofl from the macro- 

 meres and these constitute the entire ectoblast. The mesoblast forms 

 from the left posterior macromere at its fourth division, and the 



