Physical Properties of Protoplasm 153 



membrane is tightly glued to the surface of the cytoplasm, from 

 which it can be dissected only with considerable difficulty. 



The misleading optical phenomena that are involved in a study of 

 the cytoplasm are of great interest. 



It is usual for cytologists to consider the echinoderm egg a classi- 

 cal example of the alveolar structure of protoplasm. No one can 

 question the fact that beautiful round spaces with hazy, protoplasmic 

 walls in which are embedded minute granules, can be seen in such 

 eggs. Biitschli supposed these spaces to be filled with a watery fluid. 



What is the true structure of the cytoplasm of the egg of Asterias? 

 Careful dissections give a clear-cut answer to this question. 



The cytoplasm is a quiet translucent gel of comparatively high 

 viscosity; it can be drawn out into large strands, but is not cohesive 

 and elastic enough to form small threads. It can be cut into small 

 pieces with comparative ease. Fragments usually become spherical, 

 though in some cases water is slowly taken up and the mass changes 

 into the sol state. Minute granules measuring little more than one 

 micron are scattered plentifully throughout the cytoplasmic gel. 

 It has been found impossible to free these structures completely from 

 the gel in which they are embedded. They are optically more dense 

 and have a different refractive index from the surrounding living 

 substance. A part of the total mass of cytoplasm is composed of 

 what appears to be alveoli or spaces; but a careful dissection of such 

 an alveolus reveals the presence of a globule that has many of the 

 optical properties of an oil drop. Such a globule, freed from cyto- 

 plasm, does not dissolve in sea water and in a light of low intensity 

 exhibits the usual diffraction halo. The invisibility of liquid drop- 

 lets of rather high viscosity when embedded in the cytoplasm might 

 at first sight appear difficult to explain. This invisibility is due to 

 the fact that the refractive index and dispersive power of the globules 

 is very near that of sea water; also, the optical density of the cyto- 

 plasm is evidently higher than that of the globule. No diffraction 

 rings could be seen surrounding the globules when they were imbedded 

 in cytoplasm. Centrifugal force dislodges the globules, proving them 

 to be merely suspended in a living gel. The minute granules respond 

 much less readily to centrifugal force. Besides they show optical 

 properties their index of refraction is certainly higher than that of 

 the surrounding gel that ally them to highly concentrated particles 



