In 1954, when I obtained my B. Sc. (Honours School) degree in Zoology, I was asked by my advisor, the late Professor Vishwa Nath, to make comparative cytological studies of oogenesis in birds for my M. Sc. (Honours School) thesis submitted to the Panjab University in 1956. Using the classical techniques of cytology, I studied the various aspects of follicle growth and atresia in five species of birds. Professor Vishwa Nath, who himself devoted most of his life to the study of the comparative cytology of gametogenesis in animals, was so much impressed by my findings on avian oogenesis that he asked me to continue my research work for the Ph. D. degree under his supervision. During my research work on avian oogenesis, I became very much interested in further comparative study of ovarian follicles in vertebrates and thus decided to make comparative, morphological and histochemical studies of ovarian follicles in reptiles, birds and mammals. Actually, very little information was available in this regard in 1957 when I started working for the Ph. D. degree. In subsequent years, I and my numerous dedicated and highly motivated M. Sc. and Ph. D. students con- tinued to make comparative, morphological, histochemical, biochemical and physiological studies of ovarian follicles in all groups of vertebrates. In connection with the subject of this book, I would like to make special mention of my two brilliant students, namely Dr. S. K. Varma and Dr. R. K.