STRUCTURE OF THE EMBRYO AT HATCHING 



EPIDERMIS 



MESENCHYME 



133 



PROSENCEPHALON 



OLFACTORY PLACODE OLFACTORY PIT 



Fig. 76. A diagram of a cross section through the olfactory pits. 



the paired cerebral hemispheres; (2) the epiphysis, which becomes the 

 pineal body and functions in the adult as a gland of internal secretion, con- 

 trolling to some extent the development of the gonads; (3) the infundib- 

 ulum, a structure which forms the posterior lobe of the pituitary gland and 

 produces hormones which affect the contraction of muscles of the blood 

 vessels and uterus and control kidney function. 



Two thick-walled placodes invaginate and form the olfactory pits. These 

 pits will enlarge and communicate with the pharynx, while the thick walls 

 contain the neuroblasts that differentiate into the nerve fibers of the olfactory 

 nerve. 



The next section (Fig. 77) is taken from the eye region, and the large 

 thick-walled midbrain (mesencephalon) is the most prominent structure. 

 In the adult the ventral walls of the midbrain contain the nerve tracts 

 connecting the cerebrum with the posterior regions of the nervous system. 

 The dorsal wall differentiates into the optic lobes, which receive connections 

 from the optic nerves. 



The eye is located between the forebrain and midbrain. It originates as an 

 outgrowth of the forebrain, the optic vesicle, which becomes a cup-shaped, 

 two-layered structure, the optic cup. The thick layer on the inside of the cup 

 is the retinal layer, containing the neuroblasts giving rise to the optic nerve. 



