THE NEURAL TUBE AND ITS DERIVATIVES 3 1 



contact with the nasal sac to which it gives off a number of fine nerve bundles, 

 which together constitute the olfactory or first cranial nerve. At the rostral end 

 of the brain an additional nerve makes its exit from the hemisphere. It is 

 known as the nervus terminalis and can be followed forward over the olfactory 

 tract and bulb to the nasal sac (Fig. 8). 



The roof of the selachian forebrain presents a number ^structures of great morphologic 

 interest, two of which have already been mentioned, namely, the epiphysis and velum 

 transversum. The former is an outpocketing of the roof of the diencephalon; the latter 

 is an infolding and marks the line of separation between the two divisions of the prosenceph- 

 alon. Rostral to the velum the roof of the telencephalon is evaginated to form a thin-walled 

 sac, the paraphysis. The velum and paraphysis are readily identified in the mammalian 

 embryo, but become obscured in the course of later development. The morphology of this 

 region has recently been studied in great detail by a number of American investigators: 

 Minot (1901), Johnston (1909), Terry (1910), Warren (1911, 1917), and Bailey (1916). 



A good idea of the shape and connections of the various brain ventricles and 

 of the relation of the various parts of the brain to each other can be obtained 

 from a study of Figs. 9 and 11. In Fig. 13 there is indicated the location of the 

 principal sensory areas of the brain of the smooth dogfish, and the relation of 

 these areas to the corresponding peripheral nerves is apparent. The lateral 

 line components of the seventh and tenth cranial nerves are indicated in black. 



DEVELOPMENT OF THE NEURAL TUBE IN THE HUMAN EMBRYO 



In its embryonic development the nervous system of man presents some- 

 thing like a synopsis of the early chapters of its phyletic history. The neural 

 groove is the most conspicuous part of an embryo of 2.4 mm. (Fig. 14). Near 

 the middle of the body it has closed to form the neural tube, and from this 

 region the closure proceeds in both directions. The last points to close are 

 situated at either end and are known as the neuropores. The rostral end of the 

 groove shows enlargements which upon clpsure will form the brain vesicles. 

 The longer portion, caudal to these enlargements, represents the future spinal 

 cord. Except that it is flexed on itself, the brain of the human embryo of Jive 

 weeks (Fig. 15) shows a marked resemblance to the diagram of a vertebrate 

 brain without cerebral hemispheres (Fig. 7, C, D). The prosencephalic vesicle 

 is divided by a constriction into the telencephalon and diencephalon with freely 

 intercommunicating cavities. The mesencephalon is well denned and presents 

 a sharp bend, the cephalic flexure. The rhombencephalon shows signs of sepa- 

 ration into the metencephalon and myelencephalon and is slightly bent dorsally 

 at the pontine flexure. Another curvature which develops at the junction of 



