964 OF GENERATION EMBRYONIC DEVELOPMENT. 



the middle line below the mouth, forming a perfect primordial lower jaw. 

 The first proper visceral cleft is the mouth ; the second (or first of the older 

 embryologists) is the tympauo-Eustachian passage, in which the outer and 

 middle ear are developed ; the third and fourth and fifth (second to fourth 

 of authors) are early closed, but remain in Branchiate Vertebrata. 



801. Organ of Smell. According to Baer, the nasal fossae are at first dis- 

 tinct from the oral cavity, subsequently communicate with it, and are finally 

 divided into two portions, of which one becomes the respiratory portion of 

 the nasal cavity, whilst the other forms the true olfactory portion and laby- 

 rinth of the Nose. In the Chick the olfactory vesicles make their appear- 

 ance towards the end of the third day beneath the vesicles of the cerebral 

 hemispheres. The epiblast over these becomes indented to form the nasal 

 pits, which subsequently become connected with the olfactory vesicles and 

 mouth. In the Human subject two depressions, the nasal fossse, are well 

 marked at the end of the 4th week. In the 6th week these fossre com- 

 municate below with the oral cavity by a groove. At the end of the 2d 

 month the groove closes, and the labyrinth communicates with the most 

 anterior part of the oral cavity by means of two narrow orifices, which are 

 themselves rapidly closed by the formation of the palate dividing the nose 

 from the mouth, slight traces of them remaining in adult life in the Naso- 

 palatine foramen. At the 10th week the posterior nasal orifices are formed 

 on either side of the Septum. The Labyrinth is wholly developed from the 

 epiblast investing the nasal fossa?. It is fully formed at the close of the 3d 

 month, though the accessory cavities of the frontal, sphenoidal, and ethrnoidal 

 sinuses, with the antrum of Highmore, are yet absent, and are not com- 

 pleted till after the 6th month. The outer nose begins to grow out from the 

 nasal part of the primordial cranium about the end of the 2d mouth. In 

 the 3d month the nasal orifices are closed by a gelatinous plug, which disap- 

 pears after the 5th month, and seems to consist of mucus and epithelial plates. 

 As to the share of the nervous system, we have already seen that the olfac- 

 tory tract and bulb are evolutions of the 1st brain-vesicle, and nothing is 

 known in respect of the development of the nerves from the bulb. Here also, 

 therefore, the corneal layer plays an important part. It is, however, never 

 shut off' as a closed vesicle. The several steps of the development of this 

 organ are met with in the various classes of animals : the small closed fossse 

 remind us of Fishes ; the short nasal ducts opening into the anterior part of 

 the mouth, of Batrachians, etc. 



802. The extremities (Fig. 348, qq, rr) appear in Man about the fourth 

 week, as small and undivided stumps from the lateral plate ; those of the 

 upper extremity appearing first: a division into two principal parts takes 

 place in the fifth week ; one of which is broad, and shows about the fifth 

 week four indentations; the other is more cylindrical, and subdivides into a 

 fore and upper arm about the eighth week. The two extremities are very 

 similar in form up to this period, their distinctive characters only appearing 

 well marked at the third month. All the bones which enter into their com- 

 position pass through stages of development similar to those, that have been 

 already described as occurring in the bones of the vertebral column and 

 skull. In the first instance only a soft blastema or indifferent tissue appears 

 in the place of the future bone; but this gradually develops into cartilage, 

 the conversion being complete about the end of the second or the commence- 

 ment of the third mouth, and in the cartilage again points of ossification 

 make their appearance, with great though not absolute precision, for each 

 bone, not only as regards number, but also as to time and position. 1 



1 See Kollikcr, loc. cit., p. 222. 



