182 



AGE, GROWTH, AND DEATH 



in the clusters do not change for a long period. 

 Later, as the organs become differentiated, we can 

 recognise in the direct descendants of these cells, 



Germ. Cells 



mm*. 



FIG. 62. SECTION ACROSS THE POSTERIOR PART OF AN EMBRYO DOG-FISH 

 (A canthias) of 3.5 mm., to show the compact cluster of germ cells on one side. 

 The germ cells in later stages migrate from this primitive position, moving 

 singly or in small groups. Ect, ectoderm ; Md, medullary canal or primitive 

 spinal cord ; JVcA, notochord ; Mes, mesoderm ; Ent, entoderm ; X, cellular 

 strand connecting the germ-cell cluster with the yolk. (From the Harvard 

 Embryological Collection, Series 463, Section 147. Already figured by Dr. F. 

 A. Woods, but with a lower magnification.) 



which have been traced from stage to stage so that 

 their history is known with certainty, those cells which 

 in the adult we call the germ cells, and which are to 



