jj(, FINE-STRUCTURE OF PROTOPLASM II 



It is a remarkable fact that mixtures of fluids which fix different 

 types of junctions seem to effect the best fixation (seep. i6i), whereas, 

 with the exception of formalin, pure compounds produce poor 

 fixation. 



A special feature of great importance is the occurrence of beaded 

 chains observed in the electron microscope. Bessis and Bricka (1948) 

 have described such microfibrils (of ^^ 500 A diameter) in the cyto- 

 plasm of thrombocytes, Matoltsy, Gross and Grignolo (195 i) in 

 the vitreous body of cattle eyes, and Lehmann (195 1) observes similar 

 chains in the cytoplasm of Amoeba. Sheaves of such beaded chains 

 with knots of 600 A diameter occur in liver cells; Bernhard, 

 Gautier and Oberling (195 i) have shown that these beaded fibrils 

 belong rather to the ergastoplasm subject to metabolic changes than 

 to the mechanical cytoplasmic framework. In the egg of Tubifex, fibrils 

 carrying knots of about 0.15 /i diameter have been found (Lehmann 

 and Biss, 1949); these fibrils form the ground-plasm in which the 

 microscopic yolk granules (2 ^t) are suspended. The knots (0.15/^) 

 reach microscopic dimensions and are identical with the chromidia of 

 Hertwig found in the sea-urchin and Tiibifex eggs. They contain 

 ribonucleic acid (Monne, 1946a), The protoplasmic fibrils appear to 

 be segmented by the chromidia and display for that reason a micro- 

 scopical structure similar to the chromatids (see p. 225). 



Monne (1948) identifies these chromidia with the microsomes, be- 

 cause both contain ribonucleic acid (Feulgen negative, UV absorption 

 at A = 260 vafjL, stainable with pyronin), which differentiates them 

 from the mitochondria. However, such an identification must be dis- 

 carded from a morphological view, because the chromidia are im- 

 movable bodies fixed on a beaded microfibril, whereas the microsomes 

 are corpuscularly dispersed free and independent particles. 



In the gelated state cytoplasm has some continuous structure and, 

 given the chemical composition of the cytoplasm (p. 140), it must be 

 a protein gel. Protein molecules can aggregate to a framework in 

 different ways. 



a. Globular molecules or composite submicroscopic particles may 

 associate to form beaded chains (Fig. 5 la, p. 66). If these chains 

 become sufficiently long or branch, a framework is easily formed. The 

 gelation of gelatin belongs to this type (Joly, 1949). 



b. Expanded polypeptide chains can aggregate to form fibrils, such 



