\\ \ I |n\ OF PI . !, 43. 



PLATE 42. 



Fi gs- 747 6 - Transverse sections through the trunk of the same larva as figs. y 

 and 73. 



Fig. 74. Through the posterior part of the trunk (rather further back than 



:.?) 



l''k r -75- Through the nit >f the 'ail. 



Fig. 76. Rather further back than fig. 75. 



77. Longitudinal horizontal section through the vertebral column of a larva 

 of i i centims., passing through the level of the h.vmal .-ml, -wing the inter- 



vertebral 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 1 1 centims. in length. 



Fig. 79. Transverse section through an intervertebral region of the same larva as 

 % 7. 



Fig. 80. Side view of two trunk vertebne of an adult Lepidostms. 



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 4 3~ 45- (XXIV. p. S, 

 N.B. The series of sections are in all cases numbered from before backwards. 



LIST OF REFERENCE LETTERS. 



a. p. Area pellucida. ep. Epiblast. c h. Notochord. gr. Germinal wall. hy. 

 Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s. 

 Primitive streak, yk. Yolk of germinal wall. 



PLATE 43. 



SERIES A, i and 2. Sections through the blastoderm before the appearance of 

 primitive streak. 



i. 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 embry- 

 onic shield. Between the cpiblast and hypobla>t are a numlwr of undifferentiated 

 cells. The figure represents considerably more than half the section. 



SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primi- 

 live streak. 



i. Section through the anterior part of the area pellucida in front of the 

 primitive streak. 2. Section through about the middle of the primitive streak. 

 3. Section through the posterior part of the primitive streak. 



