194 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



expressed in the familiar statement that " Acquired " 

 characters^ or to use a preferable expression Impressed 

 characterSj are not inherited. By this is meant that 

 characters impressed upon the soma during its life, 

 ■such as local injuries caused by wounds or disease^ or 

 changes in size of an organ brought about by increased 

 or decreased use, are not passed on to the descendants. 

 This factj at first sight remarkable, becomes less 

 puzzling when we see in Ascaris how the cells of the 

 soma become marked off at the very earliest possible 

 stage of development as a race of cells apart from 

 those of the gonad and never give rise to true repro- 

 ductive cells. The latter, the cells which will develop 

 into the individuals of succeeding generations, are in 

 fact not derived from the soma upon which the character 

 was impressed, but are rather persisting portions of the 

 ancestral reproductive substance or gonad from which 

 that soma itself was derived. 



There is indeed a continuous streak of gonad — the 

 germ track as it is called — which comes down through 

 the ages in any chain of descent and the somas or 

 bodies of successive individuals are simply shed-off or 

 side-tracked portions of this (Fig. 88). The soma is 

 not the parent of the gonad, it is merely the foster 

 parent or nurse which has charge of its portion of 

 gonad — conveying it about, protecting, and nourish- 

 ing it. 



In the last few pages we have made use of Ascaris 

 megalocephala for illustrating some of the basic facts 

 of cytology — the department of biology which concerns 

 itself with the detailed study of cells — ^relating more 

 especially to the reproductive cells. These facts are of 

 the greatest importance and we will therefore emphasize 

 them by a few additional comments. 



(i) It will be recalled that the study of the Protozoa 

 taught us to regard the nucleus of a cell as that portion 

 of its protoplasm or living substance in which is con- 

 centrated control over its living activities. 



Diagram illustrating the side-tracking of soma from gonad. The diagram represents a portion 

 of the germ-track passing through individuals of six successive generations. Each group of three 

 black circles represents the gonad of an individual, enclosed within its soma (S). 



