948 



SCIENCE 



[N. S. Vol. XXVII. No. 703 



hydranth as the more primitive form. The 

 homology of hydranth and medusa. 



The argument based on radial symmetry 

 points toward the same conclusion. 



The Cause of Dominance in Heredity and 

 Experimental Production of Variability 

 in Dominance: W. L. Tower, University 

 of Chicago. 



An Important Period in the History of the 

 Sex Cells of Bana pipiens: Bennett M. 

 AiLEN, University of Wisconsin. 



The Experimental Production of Germinal 

 Variations, Methods, Precautions and 

 Theory of their Causation: W. L. Tower, 

 University of Chicago. 



Exuviation, Autotomy and Regeneration in 



Ceratium: C. A. Kopoid, University of 



California. 



The cellulose exoskeleton or theea of the 

 dinoflagellates by virtue of its form resist- 

 ance, specific surface and specific gravity 

 is an important organ of flotation. Pro- 

 longation of the three horns and surface 

 differentiations in the form of lists, fins, 

 ribs and rugosities increase the specific sur- 

 face and thus facilitate flotation. Walls 

 thickened by age or by compensatory re- 

 generation cut off access of light to the 

 chloroplasts and also increase the specific 

 gravity. Low temperatures increase the 

 molecular friction of water and thus in- 

 crease its buoyant properties. Changes in 

 vertical location of these pelagic protozoa 

 or modifications in the temperature of the 

 circumambient medium thus demand an 

 adjustment of this organ of flotation to the 

 changed conditions of environment. 



In many dinoflagellates the exoskeleton 

 is shed periodically in toto by ecdysis 

 either with or without schizogony. In 

 Ceratium, at least in the marine species, 

 ecdysis is unknown. In schizogony of 

 Ceratium the theea is parted diagonally 

 along definite suture lines between the 



plates and each daughter cell regenerates 

 the missing half, maintaining the ancestral 

 facies throughout by compensatory regen- 

 eration. Long-continued schizogony gives 

 rise to individuals of senile aspect with 

 heavy rugose exoskeleton. Examination of 

 San Diego plankton has brought to light 

 evidence of the fact that heavily armored 

 Ceratium in warm surface waters drop off 

 this exoskeleton by exuviation, piecemeal, 

 plate by plate, and regenerate a new wall 

 of more delicate texture. 



In deeper levels or in plankton at San 

 Diego of semitropical facies autotomy of 

 the horns is frequent. This autotomy may 

 occur at any level of the horns from the 

 tip to the base. It occurs in both ant- 

 apical horns coincidently and may also 

 appear in the median apical horn. It is 

 regulatory in character, for the specific 

 type of balanced relationship of the ant- 

 apicals is approximately preserved. Autot- 

 omy is an adaptation to flotation at deeper 

 levels or in higher latitudes. 



Kegeneration of the horns after autot- 

 omy and also renewed growth of the horns 

 without autotomy result in an increase 

 in specific surface and are adaptations to 

 flotation in higher levels or lower latitudes. 

 This regeneration is usually terminal, but 

 is sometimes basal in location on the horns. 

 It is also regulatory in character, pre- 

 serving the balanced relation of sjanmetry 

 or asymmetry of the horns which is char- 

 acteristic of the species. 



Muscle Attachment to the Body Wall in 

 the Nymphs of Anax: W. A. Riley, Cor- 

 nell University. 



The alary muscles of the njrmph of this 

 dragon-fly are exceptionally favorable for 

 the study of the relation of the muscles to 

 the body wall. The hypodermal cells at 

 the point of attachment of the muscles are 

 very much elongated (about 50/iX5/i), 

 and in specimens fixed in alcohol and 



