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MR. W. K. PARKER ON THE STRUCTURE 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. 
(g) The liyoid 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 maxillae 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 ; ( b ) 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. 
(o) The auditory capsules are still distinct from the investing mass, and a plug on the 
outer cartilaginous wall of each has become marked off as the stapes. 
( d ) 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 hyaline 
cartilage, but are beginning to ossify in their centre. 
(f) 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 manubrium 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. 
(g) 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 
