CONTENTS. 



PAGE 



Part I. The embryology and hybridization of Cidaris, 



Nature and systematic position of the material , 3 



Early development of Cidaris tribuloides 5 



Rate of development of Cidaris compared with that of other Echinoids G 



Comparison with other investigations of Cidaris 9 



Formation of mesenchyme in Echinoderms 11 



Observations on hybridization of Cidaris 13 



Influence of spermatozoon on production of paternal characters 15 



The spermatozoon and fertihzation 16 



Cross-activation versus cross-fertihzation 17 



Classification of hybrids 18 



Part II. Cytology of eggs of Cidaris tribuloides and of cross-activated Cidaris eggs. 



Structure of the Cidaris egg 21 



Study of chromosomes in species-fertiUzed Cidaris eggs 23 



Chromosomes in Cidaris eggs cross-activated by Lytechinus sperm 27 



Chromosomes in Cidaris eggs cross-activated by Tripneustes sperm 29 



Elimination of chromosomes 32 



Alteration in physical characteristics of the cytoplasm of the egg by action of foreign 



sperms 34 



Nuclear enzymes and cytoplasmic substrate 35 



The binuclearity hypothesis 36 



Discussion 37 



Summary 40 



Conclusions 41 



Literature 42 



2 



