368 



SCIENCE. 



[N. S. Vol. IX. No. 219. 



of stimuli Paramecium responds with the 

 same motor reaction, in greater or less in- 

 tensity. The direction of motion after a 

 stimulus is determined by the structure of 

 the animal's body and has no relation to the 

 localization of the stimulus. Paramecia 

 are not directly attracted by any agent ; 

 they collect in the regions of certain condi- 

 tions merely in virtue of the fact that these 

 conditions cause no motor reactions, while 

 the surrounding fluid causes a motor reac- 

 tion that results in random movements, 

 which must (through the laws of chance) 

 eventually bring the animal into a region 

 where these motors cease. 



Phototaxis of Daphnia. C B. Davenport and 



F. T. Lewis. 



The problem is to determine the depend- 

 ence of the degree of phototactic sensitive- 

 ness upon preceding conditions of illumina- 

 tion. Other conditions being similar, do 

 Daphnia reared in the dark respond to a 

 fainter illumination than those reared in 

 the light? Special apparatus afforded a 

 quantitative answer to this question. Daph- 

 nia reared in half- darkness moved, on the 

 average, nearly three times as far toward a 

 light of about minimal intensity as did Daph- 

 nia reared in the light. We may conclude : 

 Those individuals reared in the dark have 

 become attuned to a lower intensity than 

 those reared iu the light. 



The minimum intensity inducing photo- 

 taxis was, in the more sensitive Daphnia, 

 0.002 candle power at a distance of 3.5 



0.002 

 meters, or „ ^, - = 0.00016 meter candles. 

 6.0 



The phototropic sensitiveness of Daphnia 

 is quite equal to the phototropic sensitive- 

 ness of the most sensitive seedlings. 



Early Development of Pennaria Tiarella. Chas. 



W. Haegitt. 



The egg of Pennaria is of relatively large 

 size and heavily yolk-ladened. In color it 

 is of a light orange or pinkish hue. It is 



of ectodermal origin and grows by an active 

 absorption of other ovarian cells. The egg 

 is discharged almost immediately upon the 

 liberation of the medusa, which takes place 

 during the evening from seven to ten o'clock. 

 Fertilization occurs very soon after the egg 

 is discharged, or possibly in some cases be- 

 fore, since in many specimens the medusae 

 are never liberated, and the eggs seem to 

 be discharged with difficulty and not in- 

 frequently exhibit segmentation phases 

 while yet within the bell of the medusa. 

 But so far as I have been able to note, the 

 sperms always gain access to the egg from 

 the outside. 



The extrusion of the polar globules is 

 only rarely to be noted, but occurs in an 

 altogether %normal way. Segmentation be- 

 gins usually within fifteen minutes of the 

 access of the spermatozoon. The first 

 cleavage is usually into fairly normal two- 

 celled forms, but seldom exactly in the same 

 way, perhaps no two eggs exhibiting the 

 same cleavage features. This is peculiarly 

 the case in all the later phases. It is abso- 

 lutely indeterminate and remarkably irreg- 

 ular and erratic. So much so was this 

 that during the first series of observations 

 the whole lot were discarded, as probably 

 for some unknown reason abnormal or 

 pathological. A second series taken the 

 next night behaved in the same way, and 

 while still thought to be somewhat abnormal 

 were followed through to the completion of 

 the irregular cleavage, and were found the 

 following morning to have become perfectly 

 normal planuliB. 



That they wei'e genuine cleavage phe- 

 nomena was conclusively proved by sections 

 of the various stages and the demonstration 

 of mitotic figures in all phases of growth 

 and decline. 



Somewhat similar though incomparably 

 less marked phenomena had been noted long 

 ago by Wilson in the development of Re- 

 nilla, and by Metschnikoff in Rathhea and 



