10 ILLUSTRATIONS 



27. Reduced Chromosome Group of Male; and Spermatogenesis in 



Protenor ; 56 



28. Diploid and Haploid Chromosome Groups of Drosophila busckii and 



D. melanica (neglecta) 57 



29. Cross Between Wingless and Ebony Vinegar Fly 65 



30. Miniature Wing, Dumpy, and Miniature Dumpy 66 



31. Combs of Fowls 69 



32. Eight Chromosome Groups of Twelve Chromosomes Each of 



Trimeroiropis 77 



33. Back-cross of Male (Out of Black Vestigial by Wild) to Black 



Vestigial 81 



34. Back-cross of Male (Out of Gray Vestigial by Black) to Black 



Vestigial 83 



35. Scheme Showing the Inheritance of the X-Chromosome in 



Drosophila 84 



36. Back-cross of Female (Out of Black Vestigial by Wild) to Black 



Vestigial Male 89 



37. Back-cross of Female (Out of Gray Vestigial by Black) to Black 



Vestigial Male 90 



38. Scheme to Illustrate Double Crossing Over Between White and 



Forked 93 



39. Curve Showing the Influence of Temperature on Crossing Over Control 98 



40. Curve Showing the Influence of Temperature on Crossing Over .... 98 



41 . Diagram Showing Crossing Over of Two Chromosomes at Four-strand 



Stage and the Subsequent Opening Out of the Tetrad 101 



42. Scheme Showing the Opening Out of the Strands of the Tetrad 102 



43. Scheme Showing Crossing Over Involving Both Strands of Each 



Chromosome 103 



44. Spermatogonial Cells in the Last Phase of Division and the Following 



Resting Stages 105 



45. Cells Emerging From the Resting Stages Preparatory for the Next 



Spermatogonial Division 106 



46. Cells Emerging From Their Last Spermatogonial Division 106 



47. Formation of a Thick Thread after Synapsis and the Following 



Condensation of a Tetrad 107 



48. A Pair of Chromosomes in Conjugation 109 



