262 THE BIOLOGY OF BIRDS 



(4) A circular muscular layer, 



(5) More connective tissue, 



(6) A thick layer of branched tubular glands except in 



the funnel and the vagina, and 



(7) An internal lining of ciliated epithelium, with uni- 



cellular glands except in the anterior part of the 

 funnel. 



Ovum in Ovo. — ^An egg within an egg is not very un- 

 common in the case of fowls. According to Parker, double- 

 ness in eggs is due to an abnormal ovary, or to an abnormal 

 oviduct, or to both combined. Two ovarian follicles may 

 combine, or two ova may be liberated simultaneously, but 

 both these phenomena are rare. More frequently two ova 

 may be liberated from the ovary with an abnormally short 

 interval between them. If two ova are in the oviduct at the 

 same time a disturbed rhythm of contraction (" anti- 

 peristalsis ") may press a small enshelled egg up against an 

 egg not yet enshelled, and the two may be surrounded by a 

 second shell. Or it may be that a false egg, i.e. a mass of 

 albumen surrounded by a shell, may be pressed into close 

 contact with a normal egg not yet covered with a shell, and 

 the two may be covered by a second shell. 



Twinning, — Twin-embryos within one egg-shell are of 

 occasional occurrence in birds, but they are not always of the 

 same nature. According to E. S. Ruth (1916), three types 

 may be distinguished in ducks, {a) Dissimilar twins within 

 one egg-shell imply two or more fertilised ova within one 

 shell, or possibly a multinucleate ovum, {b) Identical twins 

 within one egg-shell result from the early separation of 

 parts of the segmenting embryonic area, {c) Belonging to 

 the same category are joined twins or duplicities, probably 

 due to a separation of parts of the segmenting embryonic 

 area (the blastoderm), but an incomplete separation. 



§ 7. Early Development 



In appropriate conditions of temperature, usually afforded 

 by the brooding bird, the fertilised egg proceeds to develop. 



