THE AMERICAN NATURALIST. [Vol. XXXVI I L 



tozoon enters the egg this peripheral layer, together with the 

 nuclear sap from the germinal vesicle, flows rapidly to the lower 

 pole, at which point the sperm always enters. The yolk is left 

 uncovered at the upper pole and here both polar bodies are 

 formed. The protoplasm at the lower pole then moves up to a 

 position just below the equator on the posterior side of the egg ; 

 and here the pronuclei meet and the first cleavage spindle 

 arises. At the close of the first cleavage the greater part of 

 the clear protoplasm moves into the upper hemisphere while the 

 gray, yolk-laden protoplasm is largely collected in the lower 

 hemisphere. In Cynthia the yellow pigment collects into a 

 brilliant crescent, just below the equator on the posterior side of 

 the egg ; in Ciona and Molgula this crescent is clear and almost 

 coloriess. Owing to its color in Cynthia the substance of this 

 crescent can be traced through the whole development until it 

 gives rise to the muscle cells and the mesenchyme of the tad- 

 pole. The gray substance of the unsegmented egg gives rise 

 chiefly to the endoderm, while the transparent protoplasm goes 

 principally into the ectoderm. In these eggs, therefore, there is 

 not only a localization of the material of the three germ layers 

 before cleavage, but all the axes of the future animal are clearly 

 established in the unsegmented egg. 



An Experimental Examination of the Germ Re^-:ions of the 

 Molluscan Egg: I':dmuncl H. Wilson, Columbia University.— 

 No abstract. 



Polarity and Axial Heteromorphosis : T. H. Morgan, Bryn 

 Mawr College. — The reversal of the polarity that is seen in 

 axial heteromorphosis appears to give us an opportunity for an 

 examination of the phenomenon of polarity itself, for, if we 

 could determine the conditions that determine this reversal we 

 might hope to gain some insight into the factors that determine 

 the polarity. It is from this point of view that I offer the 

 following observations on two species of planarians, Planaria 

 maenlata and P. Ingnbris. 



If a planarian is cut in two at almost any level a new tail 

 regenerates on the posterior end of the anterior piece and a new 



