unit uEgg;

interface

uses
  uChicken;

type
  TEgg = class
  private
    FChicken: TChicken;
  published
    property Chicken: TChicken read FChicken write FChicken;
  end;

implementation
. . .
end.

unit uChicken;

interface

uses
  uEgg;

type
  TChicken = class
  private
    FEgg: TEgg;
  published
    property Egg: TEgg read FEgg write FEgg;
  end;

implementation
. . .
end.

unit incubator;

interface

type

  TAbstractChicken = class;

  TAbstractEgg = class
    public
      property  Chicken: TAbstractChicken read GetChicken write SetChicken;
      function  GetChichen:TAbstractChicken; virtual; abstract;
      procedure SetChicken( Chicken:TAbstractChicken ); virtual; abstract;
  end;

  TAbstractChicken = class
    public
      property  Egg: TAbstractEgg read GetEgg write SetEgg;
      function  GetEgg:TAbstractEgg; virtual; abstract;
      procedure SetEgg( Egg:TAbstractEgg ); virtual; abstract;
  end;

unit uEgg;

interface

uses
  incubator
;  

type

  TEgg = class( TAbstractEgg )
    private
      fChicken : TAbstractChicken;
    public
      function  GetChichen:TAbstractChicken; override;
      procedure SetChicken( Chicken:TAbstractChicken ); override;
    published
      property  Chicken;
  end;

implementation

uses
  uChicken
;

TEgg.GetChicken:TAbstractChicken;
begin
    Result := fChicken;
end;

TEgg.SetChicken( aChicken:TAbstractChicken );
begin
    fChicken := aChicken;
end;

unit uChicken;

interface

uses
  incubator
;  

type

  TChicken = class( TAbstractChicken )
    private
      fEgg : TAbstractEgg;
      procedure DoSomethingWitnEgg;
    public
      function  GetEgg:TAbstractEgg; override;
      procedure SetEgg( aEgg:TAbstractEgg ); override;
    published
      property  Egg;
  end;

implementation

uses
  uEgg
;

TChicken.GetEgg:TAbstractEgg;
begin
    Result := fEgg;
end;

TChicken.SetEgg( aEgg:TAbstractEgg );
begin
    fEgg := aEgg;
end;

procedure TChicken.DoSomethingWitnEgg;
begin
    DoSomething( TEgg( fEgg ) );
end;

unit coop;

interface

uses
    uChicken
  , uEgg 
;  

function CreateChickenWithEgg:TChicken;

implementation

function CreateChickenWithEgg:TChicken;
var
    lChicken : TChicken;
    lEgg     : TEgg;
begin
    lChicken     := TChicken.Create;
    lEgg         := TEgg.Create;
    lChicken.Egg := lEgg;
    lEgg.Chicken := lChicken;
    Result       := lChicken;
end;