TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



105 



Passing along down the series into the pharynx region, observe the first, second, and 

 third pharyngeal pouches. Their dorsal diverticula come into contact with the ectoderm of the 

 branchial clefts and form the closing plates. 



Transverse Section through the Branchial Arches and the Eyes (Fig. 107). — 

 The section passes lengthwise through the four branchial arches, the fourth sunken in the 

 cervical sinus. Dorsad is the spinal cord with the first pair of cervical ganglia. The pharynx 

 is cut across between the third and fourth branchial pouches. In its floor is a prominence, 

 the anlage of the epiglottis. Ventral to the pharynx the ventral aorta gives off two pairs of 

 vessels. The larger pair are the fourth aortic arches which curve dorsad around the pharynx 

 to enter the descending aortce. The smaller third aortic arches enter the third branchial arches 

 on each side. A few sections higher up in the series the ventral aorta bifurcates and the right 



Spinal ganglion 



Nolochord' 



Ant. cardinal vein 



Pharynx 



Pharyngeal pouch 3- 



Aortic arch j- 



Pharyngeal potich 2 



Neural tube 

 Myotome 



Descending aorta 

 Branchial arch 4 

 Branchial arch 3 



Branchial arch 2 

 Mandible 



Optic vesicle 



Lens of eye 

 Diencephalon 



Fig. 107. — ^Transverse section through the branchial arches and eyes of a 6 mm. pig embryo. 



X 26.5. X, aortic aich 4. 



and left trunks thus formed give off the first and second pair of aortic arches. Cranially in 

 the angle between their common trunks lies the median thyreoid anlage. The anterior cardinal 

 veins are located lateral and dorsal to the descending aortae. The end of the head is cut 

 through the diencephalon and the optic vesicles. On the left side of the figure the lens vesicle 

 may be seen still connected with the ectoderm. The optic vesicle now shows a thick inner, and 

 a thin outer layer; these form the nervous and pigment layers of the retina respectively. 



Transverse Section through the Tracheal Groove, Bulbus Cordis and Olfactory 

 Pits (Fig. 108). — The ventral portion of the figure shows a section through the tip of the 

 head. The telencephalon is not prominent. The ectoderm is thickened and slightly invagi- 

 nated ventro-laterad to form the anlages of the olfactory pits. These deepen in later stages 

 and become the nasal cavities. In the dorsal portion of the section may be seen the cervical 



