736 



DEVELOPMENT OF THE HEAD. 



elaborate investigation of the whole subject (Jenaische Zeitschr., vol. vi., 1871), 

 who traced most carefully the nei-vous and diverticular elements in their develop- 

 ment, and their union with mesoblastic elements in the formation of the gland, 

 Goette next ascertained that the diverticulum from below is connected with the 

 buccal cavity and epiblast, and not with the pharynx and hypoblast, as was pre- 

 viously supposed (Archiv fur Miki-oscop. Anat., vol. ix., p. 397). The observa- 

 tions of Mihalkovics on Mammals complete the history of this point in develop- 

 ment, and will be mahily employed in the following description. 



The formation of this body may be shortly described as consisting in the meet- 

 ing and combination of two outgrowths from very different fundamental parts ; 

 one cerebral or medullary from above, and the other corneous or epiblastic (glandu- 

 lar), from below, in a recess of the cranial basis which afterwards becomes the 

 pituitary fossa (fig. 53.5, B, <f,'py). The cerebral outgrowth, the posteiior of the 

 two parts, takes place by the fonnation of a pointed projection do\\Tawards of a 

 portion of the lower medullaiy waU of the vesicle of the third ventricle, and its 

 firm adhesion to the base of the cranium. This is the commencement of the 

 infundibulum. Meanwhile, a little in front of the same place, there is projected 

 upwards from below a part of the basilar surface of the cranium, so as to 

 form a deep recess lined by the corneous layer from the back and upper part 

 of the future mouth. This recess is the commencement of the hypophysis or 

 pituitary- body in its glandular portion, which is not, as has been supposed, 

 a recess from the pharynx, seeing that it is in front of the opening which 

 is afterwards formed for the fauces. The depressed and sharpened out 

 anterior part of the notochord is directed downwards and forwards, while the 

 sac of the hypophysis is can-ied upwards and backwards, and, according to 

 Mihalkovics, the attenuated end of the chorda soon disappears from between the 

 infundibulum and the hypophysis, previous to the occurrence of the intimate 

 union which follows between these two bodies. The anterior extremity of the 

 chorda, therefore, is lost in the floor of the pituitary fossa, and the swollen or 

 dilated portion of the chorda which succeeds, and which comes then to form the 

 apparent termination, occupies the interval between the basi-occipital and the 

 basi-sphenoidal cartilages. The chorda traced back from this point, presents 

 another swelling at the junction of the basi-occipital cartilage with that of the 

 odontoid process, into which last it passes. The third swelling of the chorda lies 

 between the odontoid cartilage, and that of the body of the axis vertebra. 



FiK. 53G. 



B 



Fig. 536. — Cranium and 

 Human Embryo seen 

 (from Ecker). 



Face of the 

 from before 



A, from an embryo of about three weeks : 

 1, anterior cerebral vesicles and cerebral 

 hemispheres ; 2, interbrain ; 3, middle or 

 fronto-nasal process ; 4, superior maxillary 

 plate ; 5, the eye ; 6, inferior maxillary or 

 mandibular plate (first postoral) ; 7, second 

 plate ; 8, third ; 9, fourth, and behind each 

 of these four plates their respective pharyn- 

 geal clefts. B, from an embryo of five 

 weeks : 1, 2, 3, and 5, the same as in A ; 

 4, tlie external nasal or lateral frontal pro- 

 cess ; 6, the superior maxillary plate ; 7, 

 the mandibular ; x , the tongue ; 8, the first 

 phaiyngeal cleft, which becomes the meatus auditorius extenius. 



The base of the skull, therefore, consists of two parts, one the posterior, in 

 which the chorda is imbedded, and corresponding to the futui-e basi-occipital 

 and basi-sphenoidal parts, the other in front of this, into which the chorda does 

 not penetrate, the sphcno-ethmoidal, and which, according to the researchee of 

 Parker and Gegenbaur, is of a later fonnation, and is more immediately related 

 to the development of the face. 



