THE DOCTRINE OF IMMORTALITY 



53 



In insects also a special germinal path has been discovered. 

 The small egg of these animals is usually oval in form. The 

 French anatomist, Charles Robin, reported in 1862 that a 

 special group of cells appears soon after the conclusion of the 

 segmentation of the ovum. Balbiani showed twenty years 

 later that these pole cells, which are not to be confused with 

 the so-called polar globules or directive corpuscles, afterward 



vhg- 



Fig. 25. — Preparations from the egg of a beetle, Leptinotarsa. A, the whole 

 egg after completion of segmentation, at the posterior end one sees the accumulation 

 of the superficial sexual cells, p.z, Xso; B, two cells, X8so; bl.c, ordinary somatic or 

 blastodermic cell; ^.c, sexual cell (pole cell). C, section through an egg, Xios;B^ 

 blastodermic layer of somatic cells; #.c, sexual cells which migrate into the interior 

 of the egg, in order to enter the sexual gland proper. — After R. W. Hegner. 



pass into the sexual gland. The investigation of R. W. Heg- 

 ner^^ of Wisconsin University offers us the most exact account 

 of the history of these cells which we possess as yet. From 

 his paper the pictures in Fig. 25 have been taken. The pole 

 cells of Robin are sexual cells which separate precociously 

 from the somatic cells, and after they have completed their 

 migration, change in the sexual gland into sexual elements. 

 We know for animals as for pl-ants a physiological cause 



