THE BIOLOGY OF THE CELL SURFACE 



larvae. This observation teaches us one function of the oil 

 In this egg-cell: it is a reserve food-material utilized only in 

 the period before the young worm actively begins to ingest 

 food. A gram of fat, as is well known, is richer in energy 

 than a gram of carbohydrate or of protein. 



Lipin plays a role both in the water-holding power of 

 cells and in the cells' immiscibility with the surrounding 

 medium. It exists in the cell in other forms than oil drops. 

 So, cholesterol is said to be found in the membranes of some 

 eggs; the human red blood corpuscle likewise contains it. 

 A closely related compound, ergosterol, is nowadays much 

 discussed because of its significance in vitamin-chemistry. 

 Fat Is also associated with pigments and thus may play a 

 part in oxidation-processes. Yolk is another form in which 

 lipin occurs in egg-cells. 



Yolk in eggs appears in manifold chemical and physical 

 forms. The identification of yolk-bodies in eggs rests 

 largely upon staining reactions. Spherules In the cyto- 

 plasm which are stained dark blue or black with iron 

 haematoxylin are usually assumed to be yolk. If finally 

 they come to lie in the endoderm cells one concludes that 

 they are food material and presumably yolk. Even so, 

 these spherules show striking differences. Take, for 

 example, the yolk spheres in the eggs of two closely related 

 marine worms. Nereis and Platynereis. In the former ^gg 

 the yolk spheres are homogeneously blackened by haema- 

 toxylin; in the latter every yolk sphere is a delicately 

 reticular structure staining faintly with the haematoxylin. 

 In each case these bodies come to lie in the four cells which 

 compose the gut. 



At the beginning of the process of incubation, yolk 

 constitutes the greatest bulk of the chick's egg, whereas at 

 the time of hatching the yolk has largely disappeared. 

 This fact means, as we shall see In a later chapter, that yolk 

 has been converted into cytoplasmic substance. In the 



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