404 



Mr. W. K. Parker on the Structure 



[June 19, 



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 outgrowth 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 plate of the maxilla? and palatine bones 

 being foreshadowed by mere folds. The outer end of the cleft between 

 the first and second prasoral 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 between the second prseoral and first postoral arch. 

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

 external auditory meatus, is the cleft between the first and second post- 

 oral arches. The proximal end of the mandibular arch, therefore, 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 still 

 visible ; (b) the investing mass, the halves of which are completely con- 

 fluent, 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 

 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. 



(<?) The pterygo-palatine rods have increased in size ; they have not 

 become hyaline 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 manubrium 

 mallei. The rest of the arch is Meckel's cartilage ; outside 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 attaching itself to the outer surface of 

 the stapes and becoming the long process of the incus. The incus, thus 

 formed out of the proximal end of the hyoidean arch, becomes separated 

 from the rest of the arch by conversion of part of the arch into fibrous 

 tissue, and by the moving downwards and backwards of the proper hyoid 

 portion of the arch. A nodule of cartilage left in the fibrous connecting 

 band becomes a styliform interhyal cartilage, while the proximal end of 

 the detached arch becomes the stylo-hyal. 



(Ji) The ihyro-hyals have merely increased in size and density ; they 

 closely embrace the larynx by their upper ends. 



