

PLATE XVII 



FIG. 56. — Diagram of cranial and hyomandibular 

 skeleton of a normal trout embryo, about six weeks after 

 hatching, and of same age as the monstrosities examined. 

 The structures are supposed to be looked at from above 

 after removal of the roof cartilages by a section passing 

 horizontally through the nasal and periotic cartilages. 

 For description see p. 11, and for other references see 

 under PI. I. fig. 1. 



Fig. 57. — The roof cartilages of the skull of a normal 

 trout embryo, looked at from above. References to 

 description and illustrations as under Fig. 56. 



Fig. 58. — Diagram of cranial and hyomandibular 

 skeleton of a double monster embryo, of the type (Class 

 I.) illustrated in PI. I. figs. 4, 5, i.e. with union of the twin 

 brains at the optic lobes. As in Fig. 56, the structures 

 are supposed to be looked at from above after removal 

 of the roof cartilages by a section passing horizontally 

 through the nasal and periotic cartilages. The duplicity 

 affects anterior structures only. See also Fig. 59. 

 Description on p. 13, and other references under PI. I 

 fig. 4- 



Fig. 59. — Diagram showing the roof cartilages of the 

 double monster illustrated in the preceding figure. 



Fig. 60. — Diagram illustrating the cranial and hyo- 

 mandibular skeleton in a double monster belonging to 

 Class II., i.e. with union of the twin brains at the medulla 

 oblongata. The view is from above after removal of the 

 roof cartilages as in Figs. 56, 58. For description see 

 p. 16, and for other references PI. 1. fig. 6. 



FlG. 61. — Diagram showing from above the roof carti- 

 lages of the skull of the double monster illustrated in 

 Fig. 60. For general references see under Fig. 60. 



FlG. 62. — Diagram of neural and haemal arch cartilages 

 in a normal trout embryo, six weeks after hatching. 

 See p. 12. 



1: 

 bh. 



FlG. 63. — Typical arrangement of neural and haemal 

 arch cartilages at the region of transition from the double 

 to the single condition, in a double monster exhibiting 

 union of the twin bodies. See p. 13. 



Lettering in Pi. XVII. Figs. 56-63. 



a. Olfactory capsular cartilage. 

 Trabeculae cranii. 

 Hypohyal cartilage. (The glossohyal is omitted ; see 



PI. XVIII. fig. 64.) 

 Parachordal cartilages. 

 Is placed on the trabeculae cranii just in front of the 



pituitary space in Kig. 56, or the two pituitary spaces 



in Figs. 58, 60. 

 Is placed on the parachordals just behind the pituitary 



space in Fig. 56, or the two pituitary spaces in 



Figs. 58, 60. 

 Palato-pterygoid bar (/', inner or adjacent palato- 



pterygoid bar). 

 Anterior, middle, and posterior fontanelles. 

 Meckel's cartilage of loyyer jaw (g' inner or adjacent 



Meckel's cartilage of lower jaw). 

 Hyoid bar. 

 Haemal arch cartilage [/m' inner or adjacent haemal arch 



cartilage). 

 Hyomandibular (;', inner or adjacent hyomandibular). 

 On floor of periotic capsule (£', on floor of inner or ad- 

 jacent periotic capsule). 

 Pituitary space with external recti muscles passing 



through it. 

 Neural arch cartilage (Na\ inner or adjacent neural 



arch cartilage). 

 Notochord. 



Roof part of olfactory capsular cartilage. 

 Supraorbital bars (/. inner or adjacent supraorbital 



bars). 

 Spinal cord. 



Cartilage of roof of periotic capsule. 

 Laminar part of parachordals. 



Tegminal cartilages above the third ventricle and cere- 

 bellum respectively. 



A.A.A 



g- 



h. 

 ha. 



Na. 



Xch. 



spc. 



t. 



w. 



xy. 







