VARIATIONS IN SOMATIC CHROMOSOMES. 



ROBERT T. HANCE, 

 ZOOLOGICAL LABORATORY, UNIVERSITY OF PENNSYLVANIA. 



CONTENTS. 



Introduction 33 



The Cytological Conditions in CEnothera scintillans 34 



The Cytological Conditions in the Pig, Stts scrofa 35 



General Considerations 36 



INTRODUCTION. 



The chromosomes of the germ cells have received considerably 

 more attention than have their descendants in the body and our 

 knowledge of the behavior of these chromosomes is based on 

 comparatively few studies of somatic cytology. During the 

 process of karyokinesis in the body tissues the chromosomes may 

 be studied, thus affording a basis for a comparison of their general 

 characteristics and behavior with those of their ancestors or pre- 

 decessors in the reproductive glands. Such investigations may 

 eventually give an insight to some of the problems of develop- 

 ment, including the function or activities of the chromosomes, 

 and may also aid in strengthening the position taken in regard to 

 these bodies by students of gametogensis. 



Cytologists have demonstrated that in any given species the 

 chromosome number is constant and that in the diploid com- 

 plexes of the germ and body cells the chromosomes exist in pairs. 

 For a given species the chromosomes are frequently readily 

 recognizable by characteristic structure and behavior. This 

 numerical and morphological constancy has led to the theory of 

 the individuality of chromosomes and is now so well supported 

 by many observations that it may be said to be a part of the creed 

 of modern biology. Indeed our knowledge has reached the 

 point where numerical variations are not considered as prima 

 facia evidence against the theory, but as another manifestation 

 of an imperfectly understood organization. 



The following report deals with the chromosomes in the so- 



33 



