32 



GENERAL SKETCH OF THE CELL 



tive known types of nucleus ; but this subject is still sub jiidice 

 (p. 39). A transition from this condition to nuclei of the ordinary 

 type appears to be L^iven in the nuclei of certain flagellates (e.g. CJii- 

 Uwionas and Tracluhnotias), where the chromatin-granules are aggre- 

 gated about a nucleolus-like body, but are not enclosed by a membrane.^ 

 In considering the structure of the nucleus, as seen in sections, we 

 must, as in the case of the cytoplasm, bear in mind the possibility, or 



rather probability, that some of 

 the elements described may be 

 coagulation - products ; for the 

 nucleus is in life comj^osed of 

 liquid or semi-liquid substance, 

 and Albrecht ('99) has recently 

 shown that nuclei isolated in the 

 fresh condition will flow together 

 to form a single body. Most of 

 the main features of the nucleus, 

 both in the resting and in the 

 dividing phases, have, however, 

 been seen in life (Fig. 9), and the 

 principal danger of mistaking 

 artifacts for normal structures re- 

 lates to the finer elements, con- 

 sidered beyond. 



In the ordinary forms of nuclei 

 in their resting state the follow- 

 ing structural elements may as a 

 rule be distinguished (Figs. 6, 7, 

 10): — 



Fig. 13. — Two nuclei from the crypts of ^^ q^^e liuclcav DUmbraUC, a 



Lieberkiihn in the salamander. [Heidenhain.] ^^ -^ r i i t ^ n i • i 



well-defined delicate wall which 



The character of the chromatin-network • , i i 



(^aj/r/4r^wa//«) is accurately shown. The upper glVCS the UUClCUS a Sharp COUtOUr 



nucleus contains three plasmosomes or true -^^-^^ differentiates it clcarly from 



nucleoli; the lower, one. A few fine linin-threads . t . •> -r-^' 



{oxychromattn) are seen in the upper nucleus the SUrrOUuding Cytoplasm. ThlS 



running off from the chromatin-masses. The wall SOmctimCS StaiuS but VCiy 

 clear spaces are occupied by the ground-sub- ^|i n] ^,^,| ^aU SCarCcly be dif- 

 stance. n ./ ' j 



ferentiated from the outlying 

 cytoplasm. In other and perhaps more frequent cases, it approaches 

 in staining capacity the chromatin. 



b. The nuclear rcticuliiDi. This, the most essential part of the 

 nucleus, forms an irregular branching network or reticulum which con- 

 sists of two very different constituents. The first of these, forming the 

 general protoplasmic basis of the nucleus, is a substance known as lini7i 



1 Calkins, '98, i. 



