STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 



33 



kinds. There were said to be strongly basophilic 'nucleinic' nucleoli, 

 which became deeply stained with haematoxylin, and acidophilic 

 'plasmatic' nucleoli, which failed to take this stain; in addition, some 

 nucleoli were found to exhibit a shell of strongly stained material 

 covering a non-staining centre. These various effects can readily be 

 obtained if egg sections are treated with haematoxylin under the 

 usual histological conditions, but when more refined methods for 

 demonstrating basophilia and acidophilia are employed the nucleoli, 

 as already noted, are found to be uniformly acidophilic and not 

 basophilic at all. Clearly, affinity for haematoxylin applied by 

 classical methods cannot be taken as denoting basophilia in nucleoli, 

 but there is no obvious explanation for the different forms of 

 staining, in particular the rather striking 'shell' form. It has been 

 mentioned (p. 32) that nucleolar inclusions may be so large as to 

 reduce the nucleolar material to a mere shell, but such nucleoli are 

 comparatively rare, whereas those showing the 'shell' type of 

 staining were to be found in almost every nucleus. Some of the 

 recent observations with electron microscopy suggest the possibility 

 of a structural reason for the 'shell' type of staining : rat pronuclear 



Fig. 27 

 Electron micrograph of a golden hamster pronuclear egg. X 2,000. 



