4  SMITHSONIAN    MISCELLANEOUS    COLLECTIONS  VOL.    96 
layers.  Roonwal  (1937)  questions  whether  Nelsen  properly  identi- 
fied the  supposed  early  germ  cells  of  Melanoplus,  and  suggests  that 
they  are  probably  cardioblasts. 
After  the  successive  coelomic  cavities  have  merged  into  a  continu- 
ous coelome  by  the  dissolution  of  the  separating  walls,  the  germ 
cell  areas  of  the  splaiichnopleure  form  two  long  genital  strands,  or 
ridges.  These  ridges  in  the  young  embryo  of  Melanoplus,  Nelsen 
says,  extend  from  the  first  to  the  eighth  abdominal  segment ;  in 
Conoccphalus,  Wheeler  (1893)  observes,  germ  cells  may  lie  as  far 
back  as  the  tenth  segment,  though  ordinarily  they  extend  only  into 
the  sixth,  and  in  Locnsta,  according  to  Roonwal  (1937),  they  can 
be  traced  back  to  the  tenth  segment.  As  each  ridge  enlarges  it  pro- 
trudes into  the  coelome  and  comes  to  have  the  form  of  a  free  fold 
suspended  from  the  upper  part  of  the  splanchnopleure  in  the  neighbor- 
hood of  the  cardiac  rudiments ;  in  Melanoplus  (fig.  i  E)  the  fold  sup- 
ports ventrally  a  mass  of  fat  tissue  {Ft).  A  transverse  section  of  the 
male  gonad  at  this  stage,  as  described  by  Nelsen  (1931)  in  Melanop- 
lus, shows  the  following  elements  (fig.  i  E)  :  dor  sally  is  the  mem- 
branous attachment  of  the  gonad  to  the  splanchnopleure ;  below  this 
runs  a  dorsal  strand  of  mesodermal  cells  {DStr)  ;  next  is  a  central 
mass  of  germ  cells  (GCls)  and  undififerentiated  mesoderm  cells 
(MsdCls)  ;  and  finally,  a  ventral  strand  of  mesodermal  cells  {VStr), 
which  forms  the  lower  wall  of  the  gonadial  rudiment  and  supports 
the  mass  of  fat  tissue  {Pt)  beneath  it.  The  entire  organ,  from  the 
suspensory  membrane  to  the  ventral  fat  body,  is  invested  in  a  meso- 
dermal epithelial  sheath  (ESh),  the  "outer  limiting  membrane"  of 
Nelsen. 
The  later  history  of  the  male  genital  rudiment  consists  of  a  dififer- 
entiation  of  the  inner  cellular  elements,  the  formation  of  the  defini- 
tive sperm  tubes,  or  so-called  testicular  "  follicles  "  (it  should  be 
noted  that  an  ovarial  follicle  is  a  subdivision  of  an  egg  tube),  the 
establishment  of  the  sperm  tube  ducts  (vasa  efiferentia),  and  the 
formation  of  the  gonadial  part  of  the  vas  deferens.  The  following 
account  of  the  processes  involved  is  a  brief  summary  of  the  excellent 
description  of  the  development  of  the  testis  in  Melanoplus  differcn- 
fialis  given  by  Nelsen  (1931),  which  adds  much  in  the  way  of  con- 
cise information  to  our  previous  knowledge  of  the  subject  in  other 
insects. 
At  the  beginning  of  the  post-revolution  period  in  the  development 
of  the  Melanoplus  embryo,  the  genital  rudiments  have  somewhat 
shortened,  reaching  now  only  to  the  end  of  the  seventh  abdominal 
segment.    By  the  middle  of  this  period,  the  indififerent  mesodermal 
