Mr. W. K. Parker on the Skull of the Puj. 251 



membrane which closes the first visceral cleft and is the mcliment 

 of the membraua t^Tnpani. 



(r/) The lujoid or second pair of postoral arches are in this 

 stage extremely similar to the first pair, \vith which they are 

 parallel. They are stout sigmoid rods of cartilage, which are sepa- 

 rated at their distal ends, present an incurved process at their 

 opposite extremities, and are not segm(»nted. 



{h) The thyro-hyal or third postoral arches, which correspond 

 with the first branchial of the branchiate vertebrata, are repre- 

 sented by two short cartilaginous rods which lie on each side of 

 the larynx. 



{i) The olfactory sacs are suiTounded by a cartilaginous capsule, 

 which has coalesced below with the trabecula of its side ; while, 

 within, the mucous membrane lining the capsule presents eleva- 

 tions which indicate the position of the future turbinal outgrowth 

 of the capsule. 



In this stage the posterior nares are situated at the anterior 

 part of the oral caAdty, as in the Amphibia, and the roof of the 

 mouth is formed by the floor of the skull, the palatal plate of the 

 maxillae and palatine bones being foreshadowed by mere folds. The 

 outer end of the cleft between the first and second prseoral arches 

 is the rudiment of the lachrymal duct, while its inner end is the 

 hinder nasal aperture. The gape of the mouth is the cleft be- 

 tween the second praeoral and first postoral arch. The auditory 

 passage, representing the Eustachian tube, tympanum, and ex- 

 ternal auditory meatus, is the cleft between the first and second 

 postoral arches. The proximal end of the mandibular arch, there- 

 fore, lies in the front wall, and the hyoid in the hinder wall of 

 the auditory passage. 



2. In an embryo pig, an inch in length, {a) the notochord is 

 stiU visible ; {h) the investing mass, the halves of which are com- 

 pletely confluent, has become thoroughly chondrified, and is con- 

 tinued upwards at each side of the occipital foramen to form an 

 arch over it. 



'(c) The auditory capsules are still distinct from the investing 

 mass, and a plug on the outer cartilaginous wall of each has be- 

 come marked off as the stapes. 



(cl) The hinder ends of the trabecular arches have coalesced in 

 front of the pituitary body, but they are not yet confluent with the 

 investing mass. 



(«) The pterygo-palatine rods have increased in size ; they have 

 not become hyaline cartilagt^ but are beginning to ossify in their 

 centre. 



(/) In the mandibular arch the proximal end has become some- 

 what bulbous, and is recognizable as the head of the malleus, 

 whilst the incurved process, still more prominent than before, is 

 the manubrium mallei. The rest of the arch is Meckel's cartilage ; 

 outside this a mass of tissue appears, which is converted into car- 

 tilage, rapidly ossifies, and eventually becomes the ramus of the 

 mandible. 



