TRANSVERSE SECTIONS 



113 



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 aorta.. 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 

 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 telencephalon 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. 



Spinal gang. 



Kolochord 



Ant. cardinal vein 



Pharynx 



Phar. pouch 3 



Aortic arch 5 



Phar. pouch 2 



Lens of eye 



Prosencephalon 



vesicle 



Fig. 102. — Transverse section through the branchial arches and eyes of a 6 mm. pig embryo. 

 X 2b. 5. Desc. aorta, descending aorta; Br. arch 2, 3, 4, branchial arch 2, 3 and 4; Phar. pouch 2, 3, 

 pharyngeal pouches 2 and 3; X, aortic arch 4. 



Transverse Section through the Tracheal Groove, Bulbus Cordis and Olfactory 

 Pits (Fig. 10,3). — 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 

 portion of the spinal cord, the notochord just ventral to it. the descending aorta, and ventro- 

 lateral to them the anterior cardinal veins. The pharynx now is small with a vertical groove 

 in its floor. This is the tracheal groove and more caudad it will become the cavity of the 

 trachea. The bulbus cordis lies in the large pericardial cavity. On either side the section cuts 

 through the cephalic portions of the atria. These will become larger as we go caudad in the 

 series. 



