

1920I SEIIRIZ— PROTOPLASM 361 



to the frequently damaging effect of this method by presenting 

 some experimental data. 



In working with material in a hanging drop it is often very 

 desirable to have the water film of such thinness that the material 

 is held firmly against the cover slip. The surface tension between 

 water and glass thus produced is quite sufficient to hold an active 



Enplotes 



marine ova, makin 



d 



able distortion, no harmful consequences may result, but eggs of 

 seaweeds, and echinoderms are sufficiently pliable to be readily 

 distorted, so that the protoplasm is subjected to an abnormal 

 strain which frequently causes rapid deterioration. 



In order to ascertain in a general way the harm done through 

 the flattening of marine ova by a water film, I made a series of 

 observations on the ova of the sea-urchin Tripneustes. These 

 eggs were placed in a large and much spread water droplet, which 

 was of greater depth at its center than the diameter of an egg, 

 while toward the periphery the water film gradually thinned to 

 an imperceptible depth. Ova in the center of the droplet were of 

 a normal spherical shape, while those in the peripheral film were 

 much flattened. In half an hour the viscosity of the protoplasm 

 of those eggs in deep water had risen only slightly, while the 

 viscosity of those in the peripheral film had greatly increased, in 

 an hour the spherical ova had risen in viscosity a barely perceptible 

 amount, while the flattened ones had become a firm gel. Further- 

 more, the distorted eggs tolerated less dissection before showing 

 pronounced disorganization, retained the capacity for healing a 

 wound for a shorter time, and, in a great number of cases, their 

 entire contents dispersed suddenly at the first touch of a needle. 



While the use of the surface tension of a thin water film is of 

 value in holding material for microdissection, therefore, the method 



be used cautiously. Indeed, the greatest care possible must 

 be taken to keep the material living and normal, and to become 

 familiar with those criteria which assist in ascertaining the exact 

 condition of the living substance. A full appreciation of the 

 extreme irritability of protoplasm is a prerequisite to successful 

 microdissection. 



must 



