32 



GENERAL SKETCH OF THE CELL 



tive known types of nucleus ; but this subject is still sjib judice 

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

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

 lomonas and Tracliebnonas), 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 composed 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):- 



a. The nuclear membrane, a 

 well-defined delicate wall which 



Fig. 13. — Two nuclei from the crypts of 

 Lieberkiihn in the salamander. [Heidenhain.] 



The character of the chromatin-network .11 1 ^ 



(*«..f^r<,;«a/^/) is accurately shown. The upper glVCS the UUclcUS a sharp COntOUr 



nucleus contains three plasmosomes or true a.nd differentiates it clcarly from 



nucleoli; the lower, one. A few fine linin-threads , ,. „ „ ,. ^ ^i„^,„ TU^o 



{oxychromaUn) ^re seen in the upper nucleus the SUrrOUUdmg Cytoplasm. ThlS 



running off from the chromatin-masses. The wall SOmCtimCS StainS but VCry 



clear spaces are occupied by the ground-sub- slj^htlv, and CaU SCarCCly bc dif- 



stance. & y ' ^ , . 



ferentiated from the outlymg 

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

 in staining capacity the chromatin. 



d. The nuclear reticnbim. 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 limn 



1 Calkins, '98, i. 



