SOME RECENT ADDITIONS TO OUR KNOW- 

 LEDGE OF THE GERM-CELL CYCLE 

 IN ANIMALS 



By ROBERT W. HEGNER, Ph.D. 



Assistant Professor of Zoology in the University of Michigan, Ann Arbor, Michigan, U.S.A. 



One of the most fruitful lines of research that has attracted 

 the attention of biologists is the cytological study of the here- 

 ditary substance — the germ cells. Certain stages in the history 

 of the germ cells have been especially emphasised, such as the 

 maturation of the ova and spermatozoa. Other events in the 

 germ-cell cycle have not, however, been slighted, although they 

 have been rather overshadowed by the more popular study of 

 the behaviour of the chromosomes. Ever since the publication 

 of Weismann's studies on the germ-plasm theory there has been 

 no lack of interest in the morphological continuity of the germ 

 cells. A surprising number of important contributions have 

 appeared within the past eight years dealing with the early 

 segregation of the germ cells in embryological development, 

 and it is this phase of the germ-cell cycle which I wish to discuss 

 in the following pages. 



According to the idea of the morphological continuity of the 

 germ cells as first expressed by Jager (i) and Nussbaum (2), and 

 later elaborated by Weismann (3) and others, the germ cells are 

 separated from the somatic cells at an early stage in the develop- 

 ment of the egg, and they, and they only, are able to produce 

 eggs and spermatozoa. These germ cells are protected, 

 nourished, and transported by the body in which they lie ; 

 they later separate from the body and take part in the formation 

 of new individuals, whereas the body dies. The origin of the 

 germ cells has recently been definitely determined in a number 

 of animals belonging to widely separated phyla, and while there 

 are still some groups which have been carefully studied (such 

 as the Ccelenterata and Porifera) in which an early segregation 

 of the germ cells has not been demonstrated, there is no doubt 



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