TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 



107 



the head is cut through the dieneephalon and the optic vesicles. On the left side ot the figure 

 the lens vesicle may be seen still connected wi th 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. 



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 telen- 

 ceplialon is not prominent. The ectoderm is thickened and slightly invaginated 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 



Spinal gangli 



Descending aorta 

 Branchial arch 4 

 Branchial arch j 



Pharyngeal pouch 3. 



Aortic arch 

 Pharyngeal pouch 2 



Lens of eye. 



Dieneephalon 



Branchial arch 2 



'Mandible 



Optic vesicle 



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



X 26.5. X, aortic arch 4. 



to them the anterior cardinal veins. The nasopharynx 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 cardis 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. 



Section through the Heart (Fig. 109).- Lateral to the descending aortse are the com- 

 mon cardinal veins. The right common cardinal opens into the sinus venosus which in 

 turn empties into the right atrium, its opening being guarded by the two valves of the sinus 

 venosus. The entrance of the left common cardinal into the sinus venosus is somewhat 

 more caudad in the series. The trachea has now separated from the esophagus and lies 

 ventral to it. Both trachea and esophagus are surrounded by a condensation of mesen- 



