NO.  5    :mali-:  gexitalia  of  orthopteroid  insects — snodgrass        5 
cells  of  each  gonad  begin  to  differentiate  into  connective  tissue  ele- 
ments that  form  intratesticular  partitions  among  the  germ  cells,  thus 
segregating  the  latter  into  groups  of  one  or  more  cells  each  (fig.  i  F). 
The  partition- forming  cells  are  probably  generated  from  cells  at  the 
junction  of  the  gonad  proper  with  the  ventral  cell  strand.  The 
actively  proliferating  area  here  located  is  termed  by  Nelsen  the 
"germinal  center"  (GCii).  Each  cell  group,  or  cell  nest,  thus  sec- 
ondarily isolated,  marks  the  nucleus  of  a  definitive  sperm  tube,  or 
"  follicle." 
By  the  time  of  hatching,  the  gonads  have  the  form  of  two  cords 
lying  immediately  below  the  heart,  extending  from  the.  rear  half  of 
the  third  abdominal  segment  into  the  anterior  half  of  the  sixth.  The 
germ  cells  have  multiplied  until  there  are  about  four  or  five  cells  in 
each  group.  Now  an  "  indifferent  mesoderm  cell  ",  Nelsen  observes, 
leaves  the  periphery  of  the  gonad  and  pushes  into  the  center  of  each 
germ  cell  group,  where  it  sends  out  cytoplasmic  processes  that  fill  the 
spaces  between  the  germ  cells.  This  interpolated  cell  becomes  the 
apical  cell  of  the  group  (fig.  i  F,  G,  ApCl)  ;  the  entire  nest  of  cells 
is  surrounded  by  a  capsule  of  connective  tissue  cells  (G,  CpslCl). 
The  whole  formation,  which  will  be  retained  in  the  apex  of  each 
sperm  tube,  Nelsen  calls  the  apical  complex  (ApCmplx). 
The  growth  of  the  ajMcal  complexes  causes  a  series  of  lobes  to 
appear  on  the  dorsal  surface  of  the  gonad,  which  are  the  beginnings 
of  the  definitive  sperm  tubes  (fig.  i  F,  ST).  Undifferentiated  cells 
now  grow  upward  from  the  germinal  center  against  the  lower  end 
of  each  apical  complex  (Ve),  while  at  the  same  time  the  outer 
epithelial  sheath  grows  inward  between  the  dorsal  lobes  of  the  gonad 
until  it  invades  the  germinal  center.  In  this  manner  the  young  sperm 
tubes  are  formed,  each  consisting  of  a  germarium,  which  is  the  apical 
complex,  and  of  a  duct,  or  vas  efferens  (Ve),  derived  from  the 
germinal  center,  the  whole  structure  invested  in  a  fold  of  the  outer 
epithelial  sheath  (ESh). 
The  next  phase  of  activity  in  the  gonad,  which  begins  during  the 
second  nymphal  instar,  is  the  formation  of  the  sperm  cysts,  and 
the  further  differentiation  of  internal  cellular  elements.  The  germ 
cells  surrounding  the  apical  cell,  and  in  immediate  contact  with  it, 
are  the  primary  spermatogonia  (fig.  I  G,  Spg').  During  division, 
however,  some  of  the  germ  cells  are  crowded  away  from  the  apical 
cell,  or  they  are  excluded  from  it  by  a  tangential  plane  of  division. 
These  displaced  cells  become  secondary  spermatogonia  (Spg").  As 
the  latter  leave  the  sphere  of  primary  spermatogonia  they  take  with 
them  some  of  the  capsular  cells  investing  the  apical  complex,  which. 
