THE NUCLEAR APPARATUS 



19 



a thousand times that of a glass lens. With these restrictions, a theoretical resolution 

 (if 10 m/< is possible, and in practice resolutions of 2-2 m/<, equivalent to a magnification 

 of 180,000, have been attained. For biological purposes magnifications of 10,000 to 

 50,000 are employed. The different opacities in an object recorded on the photographic 

 plate are due to the scattering of the electron beams, not to the absorption and refraction 

 of light, as in the optical microscope. The intensity of the transmitted beam, for a 

 given electron speed, is roughly proportional to the thickness and density of the material 

 examined. In a mamier analogous with staining of optical microscopic objects, certain 

 biological materials may be increased in density (i.e. electron scattering power) by impreg- 

 nation with salts of heavy metals (Mudd and Anderson 1942). The value of this method 

 is at present severely limited by the necessity for completely dry specimens for examina- 

 tion, since the microscope works only in a high vacuum. (For a review of the recent 

 achievements of electron microscopy, see Mudd and Anderson 1944.) 



The great majority of the studies referred to in this section have, however, been carried 

 out by the direct observation of stained preparations, and have therefore been subject 

 to the narrower optical limitations considered above. As against these limitations must 

 be set the advantage that, in 

 preparations of this kind, we can 

 study the affinity of different cell 

 constituents for certain special 

 stains. In most, if not in all, 

 instances, staining has been pre- 

 ceded by some form of fixation, 

 so that the possibifity of proto- 

 plasmic changes due to heat, or 

 other fixing agents, must be 

 borne in mind. 



The Nuclear Apparatus. — 



One of the most controver- 

 sial questions in regard to the 

 structure of the bacterial cell 

 is the presence or absence of 

 a nucleus, and its nature if 

 present. 



It is impossible to discuss 

 at all fully the many conflict- 

 ing statements which have 

 been made, and the evidence 

 on which they have been based. Much of this evidence indeed is of little value 

 since it has been obtained by faulty or inadequate technique. Those who desire 

 fuller information on this subject may be referred to the papers of Dobell (1911), 

 Guilliermond (1907) and Hollande (1934), and to the reviews of Knaysi (1938) and 

 Lewis (1941). 



Certain of the earlier workers regarded bacteria as cells possessing no nuclei. This 

 nihilist view perhaps expressed the difficulty of demonstrating bacterial nuclei b}' methods 

 that depend upon the staining of chromatin. Chromatin, however, is not the essential 

 hereditary material, and its absence does not in any case establish the absence of a nucleus. 

 Moreover, as Lindegren (1935) points out, if the existence of some form of nuclear appar- 

 atus is denied, an explanation is required for the constancy of transmission of multiple 

 hereditary characters in bacteria, by a mechanism different from that in most other hving 

 forms. The hypothesis of some form of nuclear apparatus has at least the prior claim 

 on our attention. 



Fig. 4. — B. megatherhim. 



Unstained, dark-ground illumination, photographed with 



ultra-violet light ( X 2,500). 



