METENCEPHALON 

 MESENCEPHALON 



V GANGLION 



OTOCYST 

 MYELENCEPHALON 



VII, VIM GANGLIA 



Fig. 146. Section through the otocyst and mesencephalon. Such a section, as 

 can be seen from Figure 145, also cuts through the metencephalon and myelen- 

 cephalon. An interesting segmental structure of the brain is revealed in the walls 

 of the myelencephalon. Regular constrictions mark off segments of the brain. 

 These are called neuromeres. Near the otocysts are the primordia of the seventh 

 and eighth cranial nerves. Two dense masses of tissue on either side of the 

 myelencephalon contain the neuroblasts for the formation of the fifth cranial 

 nerve, the trigeminal. The eighth cranial nerve is the acoustic nerve ; the seventh 

 is the facial nerve. 



VISCERAL POUCH 



RATHKE'S POUCH 

 PHARYNX 



VISCERAL GROOVE 

 3RD AORTIC ARCH 

 DORSAL AORTA 



ANTERIOR CARDINAL VEIN 



ANTERIOR 

 CARDINAL VEIN 



DIENCEPHALON 



ORAL CAVITY 



THYROID SAC 



Figs. 147 and 148. Sections through the hypophysis and the thyroid. This 

 is just about the level where the Rathke's pouch is connected with the oral cavity 

 (Fig. 148). The first, second, and third aortic arches are present and the con- 

 nections of the third pair with the dorsal aorta are shown. The two brain sec- 

 tions are the myelencephalon and the mesencephalon. In Figure 148 Rathke's 

 pouch opens into the oral cavity. The section of the brain adjacent to the pouch 

 is the infundibulum of the diencephalon. The thyroid develops as an evagina- 

 tion of the floor of the pharynx and at this stage is a sac still connected with 

 the pharynx. Later it breaks off and forms a solid thyroid gland. In both figures 

 note the dorsal aorta, the anterior cardinal veins, and the numerous small vessels 

 in the head region. These small vessels are the branches of the carotid arteries. 



224 



