THE DEVELOPMENT OF THE FOWl/S SKULL. 139 



ventricle the rudimentary pineal gland (e) makes its appearance 

 in the middle line. From the middle of the lower wall grows 

 out a process, the infundibuluni, terminating in a glandular 

 appendage, the pituitary body, which last is lodged in the deep 

 fossa situated in the floor of the anterior division of the skull, 

 immediately in front of, and beneath, the termination of the 

 notochord (Fig. 57, B, D, cl). 



The three pairs of sensory organs appertaining to the higher 

 senses, — the nasal sacs, the eyes, and the ears, — arise as simple 

 caecal involutions of the external integument of the head of the 

 embryo. That such is the case, so far as the olfactory sacs 

 are concerned, is obvious ; and it is not difficult to observe that 

 the lens and the anterior chamber of the eye are produced in 

 a perfectly similar manner. It is not so easy to see that the 

 labyrinth of the ear arises in this way, as the sac resulting from 

 the involution of the integument is small, and remains open 

 but a very short time (Fig. 57, C, b). But I have so frequently 

 verified Huschke's and Remak's statement that it does so arise, 

 that I entertain no doubt whatever of the fact,* The outer 

 ends of the olfactory sacs remain open, but those of the ocular 

 and auditory sacs rapidly close up, and shut off their contents 

 from all direct communication with the exterior. The olfactorv 

 nerve is developed from the anterior division of the anterior cere- 

 bral vesicle. The optic nerve is primarily developed from the 

 posterior division of that vesicle, its connection with the middle 

 vesicle (which eventually gives rise to the corpora quadrigemina) 



vesicle of the third ventricle ; a, rudiments of the eyes and optic nerves ; b, of the 

 ears ; g, of the olfactory organs ; d, the infundibuluni ; e, the pineal gland ; c, proto- 

 vertebrae; h, notochord; 1, 2, 3, 4, 5, visceral arches; V, VII, VIII, the 

 trigeminal, portio dura, and eighth pair of cranial nerves ; k, the fronto-nasal process ; 

 I, the maxillary process ; x, the first visceral cleft. 



A, B, upper and under views of the head of a chick at the end of the second day. 



C, side view at the third day. 



D, side view at sevent3 T -five hours. 



E, side view of the head of a chick at the fifth day, which has been subjected to 

 slight pressure. 



F, head of a chick at the sixth day, viewed from below. 

 F 1 , the cartilaginous cranium of the same. 



V, pituitary space ; tr, trabecule ; Qu, quadrate cartilage ; Sc, semicircular canals ; 

 Co, cochlea. 



G, head of a chick at the seventh day, from below. 



* Sec also Kolliker's " Entwiokelimgs Geschiolito," p. 300, et seq. 



