io6 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



Section through the Myelencephalon and Otocysts of a 6 mm. Embryo (Fig. 106). 

 As the head is bent nearly at right angles to the body, this section passes lengthwise 

 through the myelencephalon. The diencephalon is cut transversely. The cellular walls 

 of the myelencephalon show a series of six pairs of constrictions, the neuromeres. Lateral 

 to the fourth pair of neuromeres are the otocysls, which show a median outpocketing at the 

 point of entrance of the endolymph duel. The ganglia of the nn. trigeminus, facialis, acus- 

 ticus, and the superior ganglion of the glossopharyngeal nerve occur m order on each side. 

 Sections of the anterior cardinal vein and its branches show on the left side. Ventral to 

 the diencephalon are sections of the internal carotid arteries. 



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. 



Fourth ventricle' 



Neitr. 6 

 Gang.juglare n. 10 



Near. 5 



Otocyst 

 Near. 4 

 Near. 3 

 Near. 2 



Near. I 



Int. carotid artery 

 Diencephalon' 



Myelencephalon 



ang. superius n. 9 



1 nt. cardinal vein 

 'Gang, acusticum n. 8 

 'Gang, geniculi n 7 



'Gang.setnilunare n. 5 

 Ant. cardinal vein 



Ant. cardinal vein 



FIG. 1 06. Transverse section through the myelencephalon and otocysts of a 6 mm. pig embryo. 



X 26.5. Neur. 1-6, neuromeres 1-6. 



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 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 Cra- 

 nially, in the angle between their common trunks, lies the median thyreoid anlage. The 

 anterior cardinal veins are located lateral and dorsal to the descending aorta:. The end of 



