I>EFK< rivji i-i.i TECS I.ARV.E. 391 



may show. In ,my case their observations as to the variability 

 in the number of micromeres agree with my own in t he develop- 

 ment of the severed blastomeres of the two cell stage of Arbacia. 

 If the eggs are similar, it would seem to be indicated clearly that 

 the localization of material which later forms the skeleton occurs 

 between tertilixation and the cut t ing through of the first cleavage. 



DIRECT DETERMINATION- <>i- THE RELATION OF IDE E<,(, Axis 



TO THE FIRST Cl.EAVACE Pl.ANE. 



The interpretation indicated by the data presented above, 

 namely that there is a stratified layer of skeleton forming material 

 at right angles to the egg axis which may be cut at any angle by 

 the first cleavage and unequally distributed to the first two 

 blastomeres, is obviously susceptible of a direct test. It is 

 possible to determine the egg axis, using the micropyle as a 

 pointer, by the use of a suspension of Chinese ink in the sea 

 water. The angle which the first cleavage plane makes with this 

 axis can then be noted, the membrane removed, and the blasto- 

 meres separated. In practice however this is an exceedingly 

 difficult and time-consuming operation. In Arbacia at least it 

 is often difficult to make out the micropyle with certainty, and 

 the large number of blastomeres which fail to develop to the 

 larval stage after membrane removal and separation usually make 

 the laborious determination valueless. I have not been able to 

 carry it out in enough cases with Arbacia to make the results of 

 any significance. The method was not tried with the eggs of 

 Echinarachnius, which appears to be a more favorable species for 

 this work. In future work I hope to make this determination 

 in a larger number ot cases. 



The relation ot the egg axis to skeleton forming or other 

 material localized in the egg might be tested in the same way by 

 \ on I'bisch's method also. The primary axis might be deter- 

 mined, ind with this actual!} in \ lew, a sector ot [he egg a 1 right 

 angles io it might be stained with the purple dye Von I bisch 

 has apparently not made this determination, prob.ibly because ol 

 the difficulties involved. 



27 



