Fig. 68. Side view of the tail of a larva, 21 minims. in length, dissected so as to shew the structure of the skeleton. Fig. 69. Longitudinal horizontal section through the vertebral column of a larva, 5.5 centims. in length, on the level of the hæmal arches, shewing the intervertebral rings of cartilage continuous with the arches, the vertebral constriction of the notochord, &c. Figs. 70 and 71. Transverse sections through the vertebral column of a larva of 5.5 centims. The red represents bone, and the blue cartilage. Fig. 70. Through the vertebral region, shewing the neural and hæmal arches, the notochordal sheath, &c. Fig. 71. Through the intervertebral region, shewing the intervertebral cartilage. Figs. 72 and 73. Transverse sections through the trunk of a larva of 5.5 centims. to shew the structure of the ribs and hæmal arches. Fig. 72. Through the anterior part of the trunk. Fig. 73. Through the posterior part of the trunk. PLATE 42. Figs. 74-76. Transverse sections through the trunk of the same larva as figs. 72 and 73. Fig. 74. Through the posterior part of the trunk (rather further back than fig. 73). Fig. 75. Through the anterior part of the tail. Fig. 76. Rather further back than fig. 75. Fig. 77. Longitudinal horizontal section through the vertebral column of a larva of 11 centims., passing through the level of the hæmal arches, and shewing the intervertebral constriction of the notochord, the ossification of the cartilage, &c. Fig. 78. Transverse section through a vertebral region of the vertebral column of a larva 11 centims. in length. Fig. 79. Transverse section through an intervertebral region of the same larva as fig. 78. Fig. 80. Side view of two trunk vertebræ of an adult _Lepidosteus_. Fig. 81. Front view of a trunk vertebra of adult. In figures 80 and 81 the red does not represent bone as in the other figures, but simply the ligamentum longitudinale superius. EXPLANATION OF PLATES 43-45. (XXIV. p. 854.) N.B. The series of sections are in all cases numbered from before backwards. LIST OF REFERENCE LETTERS. _a.p._ Area pellucida. _ep._ Epiblast. _ch._ Notochord. _gr._ Germinal wall. _hy._ Hypoblast. _m._ Mesoblast. _o.p._ Area opaca. _pr.g._ Primitive groove. _pvs._ Primitive streak. _yk._ Yolk of germinal wall. PLATE 43. SERIES A, 1 and 2. Sections through the blastoderm before the appearance of primitive streak. 1. Section through anterior part of area pellucida in front of embryonic shield. The hypoblast here forms an imperfect layer. The figure represents about half the section. 2. Section through same blastoderm, in the region of the embryonic shield. Between the epiblast and hypoblast are a number of undifferentiated cells. The figure represents considerably more than half the section.