REGENERATION AND DEATH 181 



turtle and frog. 1 It is really essentially a very simple 

 thing. Nature seems to take some of the cells which 

 are in the primitive condition, with the protoplasm 

 still undifferentiated and the nucleus of the embryonic 

 or simple organisation, and hold them apart from the 

 rest of the body ; not separating them so that they 

 come off and leave the body, but so that they have a 

 different history, so that they escape the change which 

 the other cells of the body must pass through. These 

 cells of a simpler character, which have been named 

 germ cells or sex cells, are gathered together, kept 

 asunder for a while from all the other cells of the 

 body, and never allowed to share in the development 

 of the other cells which form the body proper. For 

 instance, Dr. Woods discovered that in the develop- 

 ment of the dog-fish, very early, before any organs 

 exist, the germ cells are formed into a cluster (Fig. 

 62). They lie by themselves, are easily recognised 

 under the microscope, and they have obviously the 

 primitive character which I have endeavoured to ex- 

 plain to you, and which they long retain. Meanwhile, 

 as development progresses, all the remaining cells 

 all those not part of these clusters pursue their 

 proper careers, become differentiated ; but the cells 



1 In birds, although closely related to reptiles, the relations are less clear than 

 in turtles. Germ cells in an early stage (chick of two days' incubation with 

 26-30 primitive segments) occur in the rudiment of the wall of the intestine 

 (splanchnopleure), as they do secondarily in the turtle, and from there migrate 

 during later stages as in the turtle into the sexual glands. Unfortunately their 

 earlier history has not been traced. See W. Rubaschkin, Ueber das erste 

 Auftreten und Migration der Keimzellen bei Vogel-embryonen, Anat. Hefte. 

 Erste Abth., xxxv., pp. 241-262, Taf. 1-3 (1907). 



