1. E.W. van Lennep and L. M. Madden. Z. Zellforsch. 66 (1965) 365.
2. M. C. McClellan, J.H. Abel and G.D. Niswender. Biol. Reprod. 16(1977) 499.
3. P. Banon, D. Brandes and J.K. Frost. Acta Cytol. 8 (1964) 416.
4. D. Balmer. J. Anat. Lond. 98 (1964) 313.
5. E.C. Adams and A. T. Hertig. J. Cell Biol. 41 (1969) 696.