208 STUDY OF PIG EMBRYOS. 



followed along until it passes over into the oesophagus, Oe, which, however, is not 

 well shown, as the section passes through it away from the true median plane. 

 Between the oesophagus and the anlage of the epiglottis is a mound of tissue, La, 

 which represents the lateral wall of the developing larynx. The mound is sepa- 

 rated from the anlage of the epiglottis by a deep notch. In the median plane the 

 mound is filled with entoderm which forms a wide plate through which there is 

 only a narrow opening leading down into the trachea. Finally, we see from the 

 base of the mandible the somatopleure, Sow., extending off to form the boundary 

 of the pericardial chamber. The figure also includes a presentation of the infe- 

 rior maxillary vein, V. m.x. i, and of the thyroid gland, Thyr, which immediately 

 overlies the main trunk of the ventral aorta. This aorta gives off on either side 

 of the pharynx three principal branches, of whch the smallest is the base of the 

 carotid and corresponds to the third aortic arch. The second and third branches 

 are much larger and correspond to the third and fourth aortic arches. The pul- 

 monary aorta, P. Ao, is already separated from the main aorta of the bod)'. 



Sagittal Sections of the Head through the Principal Ganglia. — The section (Fig. 

 124) is to one side of the median plane. It exhibits the optic nerve, the trigem- 

 inal, acustico-facial, petrosal, jugular, and nodosal ganglia; but, on the other hand, 

 exhibits little of the brain, there being only a shaving from the lateral wall of the 

 fore-brain, H, and a section of the widest part of the hind-brain which shows the 

 cavity or lateral recess, R. L, of the fourth ventricle. The auditory vesicle is 

 cut, Ot. It is formed by a layer of epithelium derived from the ectoderm, 

 although now not connected with the overlying part of the epidermis by the in- 

 vagination of which the otocyst is developed. It shows a narrow, upward pro- 

 longation, the anlage of the ductus endolymphaticus (compare Fig. 127). The 

 epithelial otocyst lies in a line with the great cephalic ganglia and occupies 

 its invariable and permanent position behind the acustico-facial ganglion, 

 Ac. F, and in front of the glosso-pharyngeal, G. petr. The position of the otocyst 

 makes it an invaluable landmark in the study of sections of the head. Only the 

 lateral portion of the pharynx, Ph, appears. It forms a wide, almost slit-like 

 diverticulum, from which extend further laterally the first and second entoder- 

 mal gill pouches. In the figure can be seen a small depression extending down- 

 ward from the oesophageal or posterior end of the pharynx. This depression 

 marks the beginning of the second cleft. Nothing is seen of the third and fourth 

 clefts in this section, as they both lie nearer the median plane. The pocket or 

 diverticulum of the cervical sinus, Cerv. S, lies near the ganglion nodosum, G. 

 noil. From its appearance it might easily be mistaken for the section of a gill 

 cleft, but it is in reality lined not by entoderm, but by ectoderm, and its cavity 

 can be easily traced through the series of sections of the exterior of the embryo 



