THE STRUCTURE OF THE FERTILISED EGG 33 



development, the egg cytoplasm has no, or at most very little, 

 spatial multiplicity. 



The application of modern methods of investigation has 

 considerably increased our insight into the structure of the 

 cytoplasm in the last few years. In some respects, it was 

 shown to have the properties of a liquid. The viscosity of the 

 protoplasm of sea urchin eggs is only a few times higher than 

 that of water. In many eggs an obvious protoplasmic current 

 can be observed. Brownian movements of granular inclusions have 

 been seen in eggs, e.g. of nematode worms. Other observations, 

 however, show that cytoplasm is not a pure liquid, but that 

 it has, for instance, elastic properties. Iron particles taken up 

 by a cell can be moved by means of a magnetic field, but will 

 elastically return to their original position when the field is 

 removed. 



At present Frey Wyssling's view is generally accepted that 

 protoplasm contains a network of long polypeptide chain- 

 molecules, interconnected by side-chains. These "contacts", 

 however, are continually being broken and re-connected. The 

 meshes of the network are filled with a watery solution of salts, 

 lipids, phosphatids, etc. These dissolved substances are located, 

 in variable arrangements, around the free side-chains of the 

 polypeptide molecules. 



In some eggs, moreover, a coarser structure, of a microscopic 

 order of magnitude, is apparently superimposed upon this 

 "molecular" basic structure. This came to light especially in 

 Monne's studies (1946) of sea urchin eggs. Here the egg cyto- 

 plasm consists of a meshwork of 50-100 m^ thick fibrillae. The 

 interstices of this "spongioplasm" are filled with a more liquid 

 phase, the ''enchylema" which contains the mitochondria, yolk 

 granules, oil droplets, etc. The spongioplasm fibrillae are not 

 homogeneous, but consist alternately of granules with a strong 

 affinity for certain stains (the chromidia or microsomes), and 

 interchromidial parts. The chromidia are rich in ribonucleic 

 acid. Under the polarisation microscope, the spongioplasm 

 fibrillae proved to be negatively birefringent in longitudinal 

 direction; it was concluded that they consist of bundles of pro- 

 tein chain molecules, with lipid molecules at right angles to the 



Raven - Outline Physiologie 3 



