DEVELOPMENT OF PARA VORTEX GEMELLIPARA 499 



a dilute solution of the former acts very quickly on the yolk 

 granules, so that no clear idea of early cleavage was gained by its 

 use. Methyl green, a stain for living nuclei, left the yolk too 

 opaque to allow any satisfactory determination of its effect 

 on the embryo beneath. Acetic carmine and methyl blue, 

 besides killing the embryo, again stain the surrounding yolk 

 too strongly. 



In later stages of development, after the embryos have mi- 

 grated to the surface of the yolk or have taken it into their 

 bodies, all the stains mentioned above can be used to advantage. 



It is evident then that we must depend upon sectioned ma- 

 terial for observations of the progress of cleavage and gastrula- 

 tion. More or less uncertainty inevitably follows from the 

 various degrees of contraction which the capsules undergo 

 during fixation. (Hallez was partially successful in avoiding 

 contraction by first placing the worms in sea water containing 

 a little chloral-hydrate or cocaine.) The effect of displacement 

 upon the embryonic cells is, of course, greatest in early cleavage. 

 Earlier investigators have noted that particularly in Turbellarian 

 development there is a very weak affinity between the neigh- 

 boring blastomeres. Bresslau ('04) shows clearly in Plagi- 

 ostomum girardi and that the blastomeres normally separate 

 for distances often equal to their diameters; this phenomenon' 

 is evident as late as the twenty-four celled stage. Hallez 's 

 figures show in Paravortex cardii also a loose group of blas- 

 tomeres. Later the cells draw together to form at first a rather 

 loose, then a compact mass. 



So in the case of P. gemellipara, although the blastomeres 

 lie normally in fairly close apposition to one another, their 

 mutual bonds are easily broken by outside pressure. As a 

 result, sections often reveal isolated blastomeres crowded for 

 some distance into the surrounding yolk. 



For the reasons above noted it seems inadvisable to attempt 

 any detailed investigation of cell lineage in this organism. 

 Certain major characters, however, can be consistently observed. 



a. Micromeres and macromeres.^ The first cleavage division 

 of the egg of Paravortex gemellipara, as of P. cardii, gives rise 



lOURN'AL OF MORPHOLOGY, VOL. 27, NO. 2 



