154 COMPARATIVE ANATOMY 



of cortical gray matter is only distinctly differentiated from Rep- 

 tiles onwards. No regular series of gradations can, however, be 

 traced in this respect in the various groups. 



Connecting the two lateral halves of the fore-brain are certain 

 transverse bands .of nerve-fibres or commissures. An anterior 

 commissure is present in the posterior region of the secondary fore- 

 brain, a middle in the primary fore-brain, and a posterior in 

 the anterior part of the mid-brain. In addition to these, others 

 may be developed between the hemispheres, but only attain im- 

 portance in Mammals : they are known as the corpus callosum and 

 ihefornix. 



The outer surface of the hemispheres in all Vertebrates below 

 the Mammalia is more or less smooth : in the latter Class, convolu- 

 tions (gyri) separated by fissures (sulti) may be present. The 



FIG. 125. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE SKULL AND 

 BRAIN OF AN (IDEAL) VERTEBRATE EMBRYO. (In part after Huxley.) 



Be, basis cranii ; Ch, notochord ; SD, roof of skull ; NH 1 , nasal cavity ; VH, 

 secondary fore-brain (prosencephalon), showing the corpus striatum (Cs) at the 

 base, and the olfactory lobe (Off) anteriorly ; ZH, thalamencephalon (primary 

 fore-brain), which has given rise dorsally to the pineal body (epiphysis, Z), 

 and ventrally to the infundibulum (/), to which the pituitary body (hypo- 

 physis, H] is attached : anterior to this is seen the optic nerve (Opt], arising 

 from the optic thalamus ( Tho) ; HC, posterior commissure; MH, mid-brain 

 (mesencephalon) ; HH, cerebellum (metencephalon, secondary hind-brain) ; 

 NH, primary hind-brain (myelencephalon) ; Cc, central canal of spinal cord. 



convolutions consist of folds of the gray cortical substance, which 

 cause a greater or less increase of the superficial area. 



From the thalamencephahn, the ventricle of which is walled-in 

 anteriorly by the lamina terminalis, the following structures arise 

 (Fig. 125) : the optic thalami, formed as thickenings of its lateral 

 walls; the primary optic vesicles, arising as paired ventro-lateral 

 outgrowths from \vhich the optic nerves and retina are derived 

 later ; the pineal apparatus, developed as tube-like outgrowths of 

 thereof; and finally, the infundibulum, formed as a funnel-like 

 extension of the floor, together with a part of the pitnit&ry body 

 (hypophysis). The other portion of the pituitary body arises by a 

 gradual pinching off of the epithelium of the primary oral involu- 

 tion (stomodceum, p. 5, and Fig. 126), which gives rise to a 

 gland-like structure, and other parts (saccus vasculosus, &c.) arise 

 in close connection with it. 



