96 



LEPIDOSTEUS. 



U:-- 



A great advance has taken place in the development of the vis- 

 ceral clefts and arches. The oral region is bounded behind by a well- 

 marked mandibular arch, which is separated by a shallow depression 

 from a still more prominent hyoid arch (fig. 65, hy). Between the 

 hyoid and mandibular arches a double lamella of hypoblast, which 

 represents the hyomandibular cleft, is continued from the throat to 

 the external skin, but does not, at this stage at any rate, contain a 

 lumen. 



The hyoid arch is prolonged backwards into a considerable oper- 

 cular fold, which to a great extent overshadows the branchial clefts 

 behind. The hyobranchial cleft is widely open. 



Behind the hyobranchial cleft are four pouches of the throat on 



each side, not yet open 

 to the exterior. They are 

 the rudiments of the four 

 branchial clefts of the 

 adult. 



The trunk has the 

 usual compressed piscine 

 form, and there is a well- 

 developed dorsal fin con- 

 tinuous round the end of 

 the tail, with a ventral 

 fin. There is no trace of 

 the paired fins. 



The anterior and pos- 

 terior portions of the ali- 

 mentary tract are closed 

 in, but the middle region 

 is still open to the yolk. 

 The circulation is now fully established, and the vessels present the 

 usual vertebrate arrangement. There is a large subintestinal vein. 



The first of Agassiz' embryos was hatched about ten days after 

 impregnation. The young fish on hatching immediately used its 

 suctorial disc to attach itself to the sides of the vessel in which it 

 was placed. 



The general form of Lepidosteus shortly after hatching is shewn 

 in fig. 07. On the ventral part of the front of the head is placed 

 the large suctorial disc. At the side of the head are seen the 

 olfactory pit, the eye and the auditory vesicle ; while the projecting 

 vesicle of the mid-brain is very prominent above. Behind the 

 mouth follow the visceral arches. The mandibular arch {md) is 

 placed on the hinder border of the mouth, and is separated by a 

 deep groove from the hyoid arch {hy). This groove is connected with 

 the hyomandibular cleft, but I have not determined whether it is 

 now perforated. The posterior border of the hyoid arch is prolonged 

 into an opercular fold. Behind the hyoid arch are seen the true 

 branchial arches. 



Fig. 65. Embryo of Lepidosteus shortly before 

 hatching. 



ol. olfactory pit ; sd. suctorial disc ; hy. hyoid 

 arch. 



