112 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



90), and note the plane of each section and the structures which would appear in 

 Fig. 100. Sections typical of certain regions should be drawn. The various 

 structures may be recognized by referring to the figures of sections in the text, 

 and they should be traced through the series as carefully as time will allow. 



Transverse Section through the Myelencephalon and Otocysts of a 6 mm. Embryo 



(Fig. 101). — 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 



n ,7 , ■ 7 /..-.■ '■,. Jffl wl ' .. . \ Myelencephalon 



bourth ventricle i ,-j „;' ISffl, m I 



Neur. 6 

 Gang, jugular n.io 

 Neur. 5 

 Otocyst 

 Neur. 4 H 



Gang, superior n. p 



A ul. cardinal vein 

 Gang, acusl. n. S 

 """ -' — J ■ 'aJST JP^ ?fi — Gang, geniculal. n. 7 



Neur. 2 



Neur. j -P^^X- $X '[-^L Gone, semilunar. 



1 'ein 



I 'ein 



Int. carotid artery 



Prosencephalon 



Fig. ioi. — Transverse section through the myelencephalon and otocysts of a 6 mm. pig embryo. 

 X 26.5. Ant. cardinal vein, anterior cardinal vein; Gang, acusl. n.S, acustic ganglion of acustic nerve; 

 Gang, geniculal. 11.7, geniculate ganglion of the facial nerve; Int. carotid artery, internal carotid artery; 

 Neur. 1, 2, 3, 4, neuromeres 1, 2, 5, and 4. 



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

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

 ticus and the superior ganglion of the glossopharyngeal nerve occur in 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. 



Transverse Sections through the Branchial Arches and the Eyes (Fig. 102). — 

 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, 



