148 THE DISSECTION OF PIG EMBRYOS 



The early development of the face is practically the same in human 

 embryos (Figs. 150 and 370). In embryos of 8 mm. the lateral and median 

 nasal processes have formed. The maxillary processes next fuse with the 

 nasal processes, after which the median nasal processes unite. Coincident 

 with these changes the mandibular processes fuse and from them a median 

 projection is developed which forms the anlage of the chin. 



The external ear is developed around the first branchial groove by the 

 appearance of small tubercles which form the 'auricle. The groove itself 

 becomes the external auditory meatus and the concha of the ear. (For the 

 development of the external ear see Chapter XIII.) 



Epithelial ingrowths begin to separate the lips from the jaws at the fifth week 

 (Fig. 159). The inner edges of the lips at birth bear numerous villosities. The line of 

 fusion of the median nasal processes is evident in the adult as the philtrum. 



Anomalies. A common facial defect is hare Up. This is usually unilateral and on 

 the left side. It may involve both lip and maxilla. Hare lip is attributed to the failure 

 to fuse of the median nasal and maxillary processes (Kolliker), or the lateral and 

 median nasal processes (Albrecht). 



DEVELOPMENT OF THE PALATE 



This may be studied advantageously in pig embryos of two stages: (a) 20 to 25 mm. 

 long; (b) 28 to 35 mm. long. Dissections are made by carrying a shallow incision from 

 the anlage of the mouth back to the external ear on each side (Fig. 152). The incisions are 

 then continued through the neck in a plane parallel to the hard palate. Before mounting 

 the preparation, remove the top of the head by a section cutting through the eyes and nos- 

 trils, parallel to the first plane of section. Transverse sections through the snout may also 

 be prepared to show the positions of tongue and palatine folds before and after the fusion 

 of the latter (Fig. 151). 



In pig embryos of 20 to 25 mm. the jaws are close together and the 

 mandible usually rests against the breast. Shelf -like folds of the maxillae, 

 the lateral palatine processes, are separated by the tongue and are directed 

 ventrad (Figs. 151 A and 152 A). The median nasal processes also give 

 rise to a single, heart-shaped structure, the median palatine process (Fig. 

 152). In embryos of 26 to 28 mm. the mandible drops, owing to growth 

 changes, and the tongue is withdrawn from between the palatine processes 

 (Fig. 151 B). With the withdrawal of the tongue the palatine folds bend 

 upward to the horizontal plane, approach each other and fuse to form the 

 palate, thus cutting off the nasal passages from the primitive oral cavity 

 (Fig. 152 B). The primitive choanas (cf. Fig. 153), formed by rupture of 

 the membrane separating the olfactory pits from the oral cavity, now lead 

 into the nasal passages, which in turn communicate with the pharynx by 

 secondary, permanent choanaz. At the point in the median line where the 

 lateral and median palatine processes meet, fusion is not complete, leaving 



