278 FINE-STRUCTURE OF PROTOPLASM II 



it is shown that the morphologically differentiated parts of the cyto- 

 plasm fulfil different physiological tasks. 



Lehmann (1948) distinguishes the following systems in the cyto- 

 plasm of the egg of Tubifex which, during the development of the 

 germ, play a definite role of their own: the cortical layer, the animal 

 pole cytoplasm, the vegetal pole cytoplasm, the fibrillar central cyto- 

 plasm and the cytoplasm round the nucleus (Fig. 138). 



It would be of great interest to know the submicroscopic structure 

 of these different types of cytoplasm. Lehmann (1950) has started this 

 important electron microscope investigation with the following pre- 

 liminary results. The fibrillar cytoplasm consists of coarse beaded 

 fibrils carrying the chromidia and enclosing the yolk granules as 

 mendoned above. The polar cytoplasm has quite a different character; 

 it is a dense mass of globular elements of 30-100 m^t diameter. These 

 globules can associate and form a gel. As the polar cytoplasm of the 

 egg is later transferred to ectodermal and mesodermal cells, they 

 have been investigated individually. The ectodermal cells contain 

 similar globules (30-100 m^<), but the mesodermal cells produce large 

 ellipsoidal globules of the dimensions 600 x 300 m// or 300 X 200 m^. 

 It is open to discussion how these large particles evolve from the 

 smaller globules in the polar cytoplasm. From their density in the 

 electron micrograph they are thought to contain phosphorus. This 

 together with their microscopical size (ca. 0.3 fj) makes it look as 

 though they are related to the basophilic granules which characterize 

 the polar cytoplasm in the ordinary microscope (Fig. 138, p. 276). It 

 is strange that they should not have appeared on the electron micro- 

 graph of the polar cytoplasm. All these large basophilic granules with 

 a high phosphorus content are probably not structural elements at all, 

 but the seat of important metabolic processes. 



