PROGRESS OF MICROSCOPICAL SCIENCE. 141 



(c) The large oval auditory capsules lie on each side of the 

 anterior half of the investing mass, with which they are but imper- 

 fectly united : there is no indication of the stapes at this stage. 



(d) The trabecular or first pair of praeoral visceral arches enclose a 

 lyre-shaped pituitary space ; they are closely appKed together in front 

 of this space, and, coalescing, give rise to an azygous prsenasal ros- 

 trum. They are distinct from one another and the investing mass. 



(e) The jiterygo-palatine or second pair of visceral arches lie in the 

 maxillo-palatine processes, and are therefore sabocular in position. 

 Each is a sigmoid bar of nascent cartilage, the incurved anterior end 

 of which lies behind the internal nasal aperture, while the posterior 

 extremity is curved outwards above the level of the angle of the 

 mouth. The pterygo-palatine cartilages are perfectly free and distinct 

 from the first pr?eoral and from the fii'st pastoral arch. 



(/) The mandibular or first pair of postoral visceral arches are 

 stout continuous rods of cartilage which lie in the first visceral arch 

 behind the mouth. The ventral or 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 fii'st pair, with which they are parallel. 

 They are stout sigmoid rods of cartilage, which are sei^arated at their 

 distal ends, present an incurved process at their opposite extremities, 

 and are not segmented. 



(7i) The thijro-hyal or third postoral arches, which correspond 

 with the fii'st branchial of the 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 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 maxillte and 

 palatine bones being foreshadowed by mere fulds. The outer end of 

 the cleft between the first and second praeoral 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 prteoral and 

 first postoral arch. The auditory passage, representing the Eus- 

 tachian 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, 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 

 confluent, has become thoroughly choudrified, and is continued up- 

 wards at each side of the occipital foramen to form an arch over it. 



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



