u6 



LEPIDOSTEUS. 



beset with a number of processes, formed as thickenings of the 

 cpiblast. As shewn by Agassiz, these eventually become short 

 suctorial papiHae 1 . Immediately behind this disc is placed a 

 narrow depression which forms the rudiment of the mouth. 



The olfactory pits are now developed, and are placed near 

 the front of the head. 



A great advance has taken place in the development of the 

 visceral 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 hyomandibu- 

 lar 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 

 opercular 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 continuous round the end of the 

 tail, with a ventral fin. There is no trace of the paired fins. 



The anterior and 

 posterior portions of 

 the alimentary tract ol 

 are closed in, but the s<1 

 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 subintesti- FIG. 65. EMBRYO OF LEPIDOSTEUS SHORTLY 



, . BEFORE HATCHING. 



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



1 These papillae are very probably sensitive structures ; but I have not yet investi- 

 gated their histological characters. 



