DEVELOPMENT OF THE SKULL. 



65 



the gullet passes out and becomes continuous with that of the integument. In a 

 human embryo of three weeks of age all the arches are visible (Thomson). The first 

 or highest visceral arch is the first to appear, and it is likewise the first to unite with 

 its fellow of the opposite side : it forms by its lower part the outline of the lower jaw. 

 The posterior or upper part of the first branchial cleft remains as the external aperture 

 of the ear, the tympanic cavity and the Eustachian tube, while its anterior part together 

 with the whole of the other branchial clefts are filled up. Connected with the upper 

 edge of the first visceral arch, at its origin, a process is developed, the maxillary 

 lobe, which passes forwards beneath the eye, forming the side part of the face. The 

 eyes are formed in connection with the sides of the anterior cerebral vesicle, and, in 

 front of them, a quadrilateral mesial lobe, the middle frontal process, passes down and 

 forms the nose and middle part of the upper lip. At the angles of the extremity of 

 the middle frontal process are two slight lobes, the internal nasal processes of 

 Kolliker ; above these are two notches, the rudimentary nostrils, and between the 

 nostrils and eyes another pair of lobes, the external nasal processes of Kolliker, or 

 lateral frontal processes of Reichert. The maxillary lobe becomes united to the 

 internal and external nasal processes ; but between it and the latter there is left the 

 nasal duct. About the ninth week the inferior parts of the maxillary lobes, having 

 sent projections inwards, are united in the middle line and form the palate. 



Fig. 56. A. MAGNIFIED VIEW FROM BEFORE OF THE HEAD 



AND NECK OF A HUMAN EMBRYO OF ABOUT THREE WEEKS 



(from Ecker, Icones Physiol. Tab. xxix. fig. I.). 



1, anterior cerebral vesicle or cerebrum ; 2, middle ditto ; 

 3, middle or naso-frontal process ; 4, superior maxillary pro- 

 cess ; 5, eye ; 6, inferior maxillary process or first visceral 

 arch, and below it the first cleft ; 7, 8, 9, second, third, and 

 fourth arches and clefts. 



B. ANTERIOR VIEW OF THE HEAD OF A HUMAN Forrus OF 



ABOUT THE FIFTH WEEK (from Ecker, as before, fig. IV.). 



1, 2, 3, 5, the same parts as in A ; 4, the external nasal 

 or lateral frontal process ; 6, the superior maxillary process ; 

 7, the lower jaw ; x , the tongue ; 8, first branchial cleft 

 becoming the meatus auditorius externus. 



Formation of the cranium. The chorda dorsalis passes 

 into the base of the skull, as far forwards as the sphe- 

 noidal region. According to the observations of H. Muller 

 on the calf (Ueber das Vorkommen von Chordaresten, &c., 

 in Zeitschr. fur Rat. Med. 3rd series, vol. ii.), it extends 

 from the odontoid process of the axis through the basilar 

 process of the occipital bone to the b#ck of the dorsum 

 sellse, which it pierces, and is lost behind the pituitary 

 body. Before attaining the osseous state, the cranium passes 

 through a membranous and cartilaginous condition, in which 

 it is termed the primordial cranium. In the membranous 



cranium, the blastema, immediately beyond the extremity of the chor4a dorsalis, 

 presents two thick bars, the lateral trabeculce of Rathke, with a very thin part 

 in the middle line between them, corresponding to the position of the pituitary 

 body (see fig. 57, 4). These trabeculse unite in front in the ethmoidal region, and it is 

 to be remarked that in the cartilaginous stage the ethmo-vomerine cartilage or mesial 

 septum of the nose is continued directly forwards from the region containing the 

 chorda dorsalis. In the cartilaginous stage of the cranium the cartilage can only be 

 traced in the basilar parts, and corresponds in extent to the occipital bone below the 

 level of its protuberance, the petrous and mastoid portions of the temporal bone, the 

 sphenoid and the ethmoid, while the part of the wall in which the frontal and parietal 

 bones, the upper part of the occipital, and the squamous portions of the temporals 

 appear, remains membranous. The extent, however, over which the cartilage of the 

 primordial cranium extends differs in different animals, and is greater in many 

 mammals than in man (Kolliker). The nasal bones are derived from the middle 



