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 

 comviissure 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-braiu. 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 

 the fornix. 



The outer surface of the hemispheres in all Vertebrates below 

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

 tions igyri) separated by fissures {sulci) may be present. The 



■ J L ■ ■ ■ ■ 



Fig. 125. — Uiagbammatic Longitudinal Section thkough the Skull and 

 Beain of an (Ideal) Vertebrate Embryo. (In part after Huxley.) 



Be, basis cranii ; Oh, notochord ; SD, roof of skull ; NH}, nasal cavity ; YH, 

 secondary fore-biain (prosencephalon), showing the corpus striatum (C'«) at the 

 base, and the olfactory lobe ( O//) 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^ ; HG, posterior commissure ; J/i7, 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 thalamenceplialon, the ventricle of which is walled-in 

 anteriorly by the lamina terminalis, the following structures arise 

 (Fig. 12.5) : — the optic thalami, formed as thickenings of its lateral 

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

 outgrowths from which the op)tic nerves and retina are derived 

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

 the roof; and finally, the infundihulu,m,iovm.e.A. as a funnel-like 

 extension of the floor, together with a part of the pituitary 'body 

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

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

 tion {st.omodmmn, p. .5, and Fig. 126), which gives rise to a 

 gland-like structure, and other parts {saccus vasculosxis, &c.) arise 

 in close connection with it. 



