roi 



DEVELOPMENT OF THE HEAD. 



cula3 meet, and where inferior]? tlie })ituitary body, and superiorly the 

 infnndibnlum are afterwards formed. The investing mass of blastema, 

 in which the anterior extremity of the notochord is enclosed, and the 

 notochord itself, terminate here behind the pituitary fossa, or what 

 afterwards becomes that part, in a place corresponding to the dorsum selliB 

 of human anatomy. Above and behind this, the middle cerebral vesicle 

 forms the most prominent part of the cranium, which remains a charac- 

 teristic feature of this part of the embryo head for a consideral^le time. 

 Another early flexure of the cranium accompanies the development 

 of the cerebellum from the third primary vesicle, a cleft now appearing 

 behind and below the rudimentary cerebellum, in the region of the 

 fourth ventricle, and above the medulla oblongata, and this flexure is 

 necessarily attended with a convexity forwards, or another flexure in 

 the place of the pons Varolii. 



Fig. 534. 



Fig. 534. LONC.ITUPTXAL 



Section of the Human 

 Embryo at the sixth or 



SEVENTH ^VEEK 



^^ ~'~"' """ ^ '"' 1, cerebral hemispheres 



2, vesicle of the third ven- 

 ti'icle ; 3, mid-hrain ; 4, 

 cerebellum ; 5, medulla ob- 

 longata ; c7i, notochord pass- 

 ing up through the bodies 

 of the vertebrae into the 

 basis cranii and terminating 

 in the head between the in- 

 fundibulum and the sac o£ 

 the hypophysis cerebri ; s, 

 the vertebral spines ; n, the 

 spinal cord ; p, the phannix ; 

 7i, the heart ; I, the liver ; ?', the stomach and intestine; cl, the cloaca ; r, the urinary 

 bladder and pedicle of the allantois ; it, u', the umbilicus containing the vitello- 

 intestinal duct, urachus and vessels ; between i, and /, superiorly, the Wolffian body is 

 shown. 



The great cranial flexure thus marks the division between the strictly 

 basi-cranial, or occipito-sphenoidal, and the basi-facial, or spheno- 

 ethmoidal part, the chorda terminating between those two portions 

 of the cranial base, with a conical and sharp point. Here the chorda 

 is itself lient downwards and forwards, and terminates in a s]3ot 

 which corresponds to the post-sphenoid body, or dorsum sellas. 

 According to Mihalkovics, who has recently investigated the subject 

 with care (see Archiv filr j\Iikroskop. Anat., vol. xi., 1875,) in con- 

 nection with the formation of the pituitary gland in mammals and 

 birds, the chorda tapers off to a fine point in front of this spot, but 

 presents a slight swelling just at the place of the future occipito- 

 sphenoidal suture. 



The formation of the mouth, and its opening by the fauces into the 

 pharyngeal or first part of the primitive alimentary canal, are phenomena 

 of development intimately connected with the formation of the central 

 part of the cranium and sella turcica, but they are also associated with 

 the development of the face, which is next to be considered. 



Formation of the Moutli and Hypophysis cerebri. — Along with the 

 changes which accompany the formation of the principal cranial flexui-e, is 



