(i)  Dermoid  cysts  are  lined  by  epithefium  and  contain  epithelial  prod-
ucts, such  as  hair,  nails,  teelli,  sebum,  mucus,  (a)  Sequestration  dermoids
arise  from  the  inclusion  of  a  portion  of  the  epiblast  in  situations  where  embry-
onic segments  unite,  e.g.,  in  tlie  middle  hne  of  the  body  and  in  the  region  of
the  facial  and  branchial  clefts.  In  the  face  the  most  common  situation  is
just  behind  the  external  angular  process  of  ihe  frontal  bone  (orbito-nasal
cleft),  in  which  region  an  iij)ening  may  persist  in  ihe  skull  and  the  dermoid  be
connected  with  the  dura  mater,  (b)  Tubuio-dernumh  arc  those  developing
in  funclionless  duels  or  obsolete  canals,  the  most  common  situations  being
the  thyro-glossal  duct  and  the  post-anal  gut.  (c)  Ovarian  dermoids  may  con-
tain not  only  dermal  structures,  but  also  mesoblastic  structures,  such  as  bone
and  cartilage;  in  the  latter  instance  they  are  supposed  to  be  due  to  the  inclusion
of  a  bUghted  ovum  {teratoma),  (d)  Implantatiim  dermoid  {acquired  or  trau-
matic dermoid)  is  due  to  the  thrusting  of  epithelial  cells  into  the  subcutaneous
tissues,  usually  as  the  result  of  a  punctured  wound.
(a)  Blood  cysts  may  arise  from  e.xtravasation  of  blood  (hematoma).  A
second  variety  often  found  in  the  neck  b  of  doubtful  origin;  it  has  a  ihin  wall
and   communicates  with  the   interior  of  a  vein.
