STRUCTURE OF FOUR-DAY CHICKS 1 23 



entire epithelial portion of the internal ear mechanism. Nerve 

 fibers arising from the acoustic ganglion grow into the brain 

 proximally and to the internal ear distally establishing nerve 

 connections between them. There is at this stage no indication 

 of the differentiation of the external auditory meatus. The 

 dorsal and inner portion of the hyomandibular cleft which 

 gives rise to the eustachian tube and to the middle ear chamber 

 has not yet become associated with the auditory vesicle. 



The Olfactory Organs. — The olfactory organs are represented 

 in three-day and four-day chicks by a pair of depressions in the 

 ectoderm of the head. These so-called olfactory pits are located 

 ventral to the telencephalic vesicles and just anterior to the 

 mouth (Figs. 40 and 41). By growth of the processes which 

 surround them, the olfactory pits become greatly deepened. 

 The epithelium lining the pits eventually comes to lie at the 

 extreme upper part of the nasal chambers and constitutes the 

 olfactory epithelium. Nerve fibers grow from these cells to 

 the telencephalic lobes of the brain to form the olfactory nerves. 



IV. The Digestive and Respiratory Systems 



Summary of Development Prior to the Third Day. — The 



primary entoderm which gives rise to the epithelial lining of the 

 digestive and respiratory systems and their associated glands 

 becomes established as a separate layer before the egg is laid. 

 In its early relationships the entoderm is a sheet-like layer of 

 cells lying between the ectoderm and the yolk and attached 

 peripherally to the yolk (Fig. 7). The primitive gut is the 

 cavity bounded dorsally by the entoderm and ventrally by the 

 yolk (Fig. 31, A). 



Only the part of the entoderm which lies within the em- 

 bryonal area is involved in the formation of the enteric tract. 

 The peripheral portion of the entoderm goes into the formation 

 of the yolk-sac. There is at first no definite line of demarcation 

 between the entoderm destined to be incorporated into the 

 body of the embryo and that which remains extra-embryonic 

 in its associations. The foldings which appear later separating 

 the body of the embryo from the yolk, establish for the first 

 time the boundaries between intra-embryonic and extra-em- 

 bryonic entoderm (Figs. 30 and 32). 



