PLATE 3 
EXPLANATION OF FIGURES 
Figures 18 to 27 are taken from cross-sections and frontal sections of Squalus 
embryos, stages G to L, to show the relations of the ectoderm and endoderm 
in the pharyngeal region. Subsequent to the perforation of the visceral clefts 
the limits of the ectoderm and endoderm are difficult to determine and the dia- 
grams are to be understood as only approximately accurate. 
18 A cross-section of a Squalus embryo (stage G) through the first (spiracular) 
pouch. 
19 A cross-section through the same pouch (stage H). In the region of the 
future cleft the ectoderm thins out more rapidly than does the endoderm. 
20 A cross-section (stage I) through the first and second pouches. 
21 A cross-section (stage K) through the first and second pouches and the 
mouth which has broken through at this stage. . 
22 A frontal (horizontal) section of stage H, showing the endodermal out- 
pocketings to form the first, second, and third visceral pouches. 
23 A frontal section of a Squalus embryo (stage I) cut dorsal to the hypophy- 
sis, showing the formation of the first four visceral pouches. 
24 A frontal section of a Squalus embryo (stage I) at the level of the hypophy- 
sis, showing the formation of two visceral clefts and two pouches. 
25 A frontal section of a Squalus embryo (stage K) dorsal to the level of the 
hypophysis, showing the ectodermal-endodermal relations in the regon of the 
pharynx. 
26 A frontal section of a Squalus embryo (stage L) cut at the level of the 
hypophysis, showing the exent of the pharyngeal epidermis. 
27 A horizontal section of the same stage as figure 26, but cut at a higher level. 
60 
