34 



HUMAN EMBKYOLOGY. 



MID- 



CHORDA 

 DORSALIS 



and the anterior medullary velum constitute parts of the isthmus rhombencephali 



(Figs. 45, 63). 



The ventral portion of the primitive mid-brain is converted into the two 



peduncles of the cerebrum of the adult brain, and the dorsal portion is transformed 



into four rounded elevations, the colliculi or corpora quadrigemina. 



The transformations which take place in the region of the primitive fore-brain 



or prosencephalon are numerous and complicated; therefore its ventral, lateral, 



and dorsal walls require separate consideration. 



By the expansion of its cephalic (anterior) extremity is formed the secondary 



fore-brain, which becomes divided, as already explained, into the two secondary 



vesicles which are the rudiments of the cerebral hemispheres of the completed brain. 

 After the formation of the rudiments of the cerebral hemispheres, which 



constitute the tel- 

 M : i o ^ encephalon of the 



adult, the primi- 

 tive fore-brain and 

 the undivided 

 stalk of the second- 

 ary fore-brain 

 diverticulurn be- 

 come the dience- 

 phalon. 



The cephalic 

 or anterior end of 

 the diencephalon 

 is closed by the 

 lamina terminalis 

 (see p. 33), in 

 association with 

 which are subse- 

 quently developed 



FIG. 45. DIAGRAMS TO ILLUSTRATE THE ALAR AND BASAL LAMINA. In both two columns 

 cases the embryonic brain is represented in mesial section (His). which run dorSO- 



A. The different subdivisions of the brain are marked off from each other by dotted ventrally, the 

 lines, and the dotted line running in the long axis of the neural tube indicates tne co lumns of the 

 separation of the alar from the basal lamina of the lateral wall. . 



B. Medial section through the brain of a human embryo at the end of the first -11 ' \ 

 month. Dotted lines mark off the different regions and also the alar and basal tenor pillars;, and 

 laminae from each other. two transverse 

 H, Buccal part of hypophysis cerebri ; RL, Olfactory lobe ; C.STR, Corpus striatum ; Commissures, One 



A, Entrance to optic stalk ; 0, Optic recess ; I, Infundibular recess ; T, Tuber Q which connects 

 cinereum ; M, Mamillary eminence. together the two 



cerebral hemispheres and is called the anterior commissure, whilst the other is 

 the optic chiasma in which the medial fibres of the optic nerves decussate. 



From the cephalic or anterior end of the ventral wall of the diencephalon a 

 diverticulurn is projected ventrally towards the dorsal wall of the primitive mouth. 

 The ventral end of this diverticulurn becomes the posterior lobe of the hypophysis 

 (O.T. pituitary body) of the adult, the dorsal end becomes the tuber cinereum, and 

 the intermediate part is the infundibulum which connects the tuber cinereum of 

 the adult brain with the posterior lobe of the hypophysis. 



Caudal to the hypophyseal diverticuium a single elevation appears in the 

 ventral wall of the diencephalon. It is the corpus mamillare, which afterwards 

 separates into the paired corpora mamillaria of the adult brain. 



Still more caudally the ventral wall of the diencephalon takes part in the 

 formation of the substantia perforata posterior, which lies between the two 

 peduncles of the cerebrum and is partly developed from the cephalic or anterior 

 end of the ventral wall of the primitive mid-brain. 



The greater part of the dorsal wall of the diencephalon is ultimately reduced 

 to a single layer of epithelial cells, but near its caudal end a diverticuium is 

 projected dorsally. This is the epiphysis or pineal body, which remains quite 



