52 THE DOCTRINE OF IMMORTALITY 



are the sexual cells and they accomplish during the later 

 development a wonderful migration, for they move through 

 the wall of the digestive canal and then through the mesen- 

 tery until they reach the spot where the sexual gland arises. 

 We known this interesting history through the investigations 

 of F. A. Woods, 21 which were made in my laboratory. For- 

 merly one assumed that the sexual cells arose in the gland, 

 but this is probably not the case in any vertebrate. Another 

 American, B. M. Allen, 22 has greatly enlarged our knowledge 

 of the history of the sex cells in vertebrates. By the re- 

 searches of this investigator, we now know that also in the 

 turtle, the frog, and in two fishes, Amia and Lepidosteus, the 

 sexual cells may be recognized very early. They lie at 

 first far from the sexual gland into which they later migrate. 

 The paths which these cells take during their migration 

 differ for the species mentioned, Fig. 24. Several European 

 investigators have also occupied themselves with the history 

 of the sexual cells in vertebrates. In spite of the fact that 

 much remains to be cleared up, we may nevertheless assert 

 that vertebrates have special germinal paths, as they are 

 called. In other words sexual cells are held apart. They pass 

 through their development by themselves and have nothing 

 in common with the somatic cells. They do not participate 

 in the structure of the body, but remain almost like guests 

 which are cared for by the other cells. When the proper 

 time comes the sexual cells change themselves, as the case 

 may be, into male or female elements. Since we know the 

 history of these cells exactly in several cases, we are able to 

 assert that in sexual as in asexual propagation the living 

 substance continues uninterruptedly. This continuation up 

 to the origin of the sexual elements we have actually ob- 

 served. 



