328 



ME. W. K. PAEKER ON THE STEUCTUEE AND 



distal ends of these arches are not yet in contact ; the dorsal or proximal end of each is 

 somewhat pointed and sharply incurved, pushing inwards the membrane which closes 

 the first visceral cleft and is the rudiment of the membrana tympani. 



(ff) The hi/oid or second pair of postoral arches are in this stage extremely similar to 

 the first pair, with which they are parallel. They are stout sigmoid rods of cartilage, 

 which are separated at their distal ends, present an incurved process at their opposite 

 extremities, and are not segmented. 



(h) The thyrohyal or third postoral arches, which correspond with the first branchial 

 of branchiate Vertebrata, are represented by two short cartilaginous rods which lie on 

 each side of the larynx. 



{i) The olfactory sacs are surrounded by a cartilaginous capsule, which has coalesced 

 below with the trabecula of its side ; while, within, the mucous membrane lining the 

 capsule presents elevations which indicate the position of the future turbinal outgrowths 

 of the capsule. 



In this stage the posterior nares are situated at the anterior part of the oral cavity, 

 as in the Amphibia ; and the roof of the mouth is formed by the floor of the skull, the 

 palatal plates of the maxilla; and palatine bones being foreshadowed by mere folds. 

 The outer end of the cleft between the trabecula and the secondary preoral arch appears 

 to be the rudiment of the lacrymal duct, while its inner end is the hinder nasal aperture. 

 The gape of the mouth is the cleft between the second preoral and the first postoral arch. 

 The auditory passage, representing the Eustachian tube, tympanum, and external auditory 

 meatus, is the cleft between the first and second postoral arches. The proximal end of 

 the mandibular arch, therefore, lies in the front wall of the auditory passage, and the 

 hyoid in its hinder wall. 



2. In an embryo pig, an inch in length, [a) the notochord is still visible ; {h) the 

 investing mass, the halves of which are completely confluent, has become thoroughly 

 chondrified, and is continued 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 become marked ofl" as the stapes. 



((/) 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. 



{e) The pterygo-palatine rods have increased in size ; they have not become hyalipe 

 cartilage, but are beginning to ossify in their centre. 



(/) In the mandibular arch the proximal end has become somewhat bulbous, and is 

 recognizable as the head of the malleus, whilst the incurved process, still more prominent 

 than before, is the manuhrium mallei. The rest of the arch is Meckel's cartilage ; out- 

 side this a mass of tissue appears, which is converted into cartilage, rapidly ossifies, and 

 eventually becomes the ramus of the mandible. 



{(f) The proximal end of the hyoidean arch, similarly enlarging and articulating with 

 the corresponding part of the mandibular arch, becomes the incus, the incurved process 



