CONTENTS 



PAGE 



INTRODUCTION . . - vii 



BIBLIOGRAPHY - ...... xi 



CHAPTER I. DOUBLE MONSTROSITY 



A. Literature - . . . .. . 1 



B. Occurrence, records, and general outline ......... 1 



C. Classification - ......3 



D. Causation .......4 



E. Development and early growth in fishes, and in vertebrates generally 6 



F. Structure 



Early Stages of Double Monstrosity in the Salmonidae and in Eiox - 9 



Normal Advanced Embryos of Salmo fario - 11 

 Advanced Stages of Double Monstrosity in Salmo fario, etc. 



Class I. Union in head region, the twin brains uniting at the optic lobes 12 



II. Union in head region, the twin brains uniting at the medulla oblongata 15 



III. Union in pectoral region, adjacent pectoral fins not being present - 18 



IV. Union in pectoral region, adjacent pectoral fins being present, but united and 



reduced in size - 19 



V. Union by the body or tail, the united portion ending as normally in a single 



symmetrical tail - 20 



VI. Union by the body or tail, the united portion ending in a composite tail with 



various structures still doubled - - 21 



VII. Union by the yolk-sac only, or by the yolk-sac and the ventral-edge membrane 



close behind it 23 



VIII. The condition of imperfect doubling exhibited by (a) the (hemi)mesodidymi and 



(b) (hemi)katadidymi - 25 



IX. Longitudinal or parallel union 29 



Double Monstrosity in other Fishes 30 



CHAPTER II. TRIPLE MONSTROSITY 



Description of a specimen - 33 



Other instances in fishes - 35 



Mode of development in fishes 36 



Appendix on Triplicity in other vertebrates, and on Classification in Germinal Triplicity - 37 



