EXPLANATION OF DIAGRAMS. 411 



meacd cmlim appear in this figure, but the extent of the right procoelia is indicated at the 

 cephalic end of the right hemisphaera 



The porta is represented by the darli spot. 



The order of succession of the coeliae and of the segments is seen to be the same as 

 in Fig. 110. 



As in Fig. 110, the shorter transverse lines are intersegmental, and the longer ones 

 (A-H) persegmental. In most cases the latter correspond closely with those of the 

 persegraental lines in Fig. 110, but the line H is placed farther caudad, there being, in 

 the frog and Menobranchus, no considerable difference between the parts of the hemi- 

 Bpheres. 



The several coelise are named as in Fig. 110, but in place of the names of the ence- 

 phalic segments are given the names oi t\tB\T principal parts (§ 1069). 



The floor of the mesocalia is formed by the parts called crura cerebri in the higher 

 Vertebrates, and its sides and roof by the optici. Between the optici and the cerebellum, 

 proper, and perhaps belonging in part to both segments, is a thin and incurved portion of 

 the roof, the valvula (Vieussenii). 



The roof of the diacalia is variously constituted. Its caudal portion consists of ner- 

 vous tissue, which in the frog, according to Wyman (34, 11), presents commissural 

 fibers, the postcommissura (Stieda, 22, 17; Ecker, B, Abt. II, 10). The cephalic part 

 consists mostly of the membranes, but presents the thickening commonly known as coiia- 

 rinm or epiphysis or pineal gland (Wyman, 3-1:, 11), which, however, may be only an 

 indication of the place of attachment of the true conarium (§ 1084). The depressed floor 

 presents a diverticulum, the infundibulura, leading to the hypophysis or pituitary body. 

 Ventrad of the cephalic portion of the floor is a transverse band of fibers, the chiasma of 

 the optic nerves. 



The aula forms the last or most cephalic of the mesal series of cavities. Most of its 

 floor and part of its cephalic boundary is formed by the terma, of which the praecomrais- 

 sura {pros.) is really a thickening and differentiation. 



In this figure, instead of the unbroken lateral walls of the coeliae, there are seen the 

 roofs and floors of the me^al series, irregular in contour and variously constituted in dif- 

 ferent parts. The proper nervous tissue is atrophied in several places, and the coelian 

 parieties consist chiefly of the two membranes, the enveloping pia and the lining en- 

 dyma (§ 1080). 



Each of these membranes is represented by a narrow black line, while in this, as in 

 Fig. 110, the surface of the nervous tissue is represented by a heavy line. 



The metatela, or roof of the metacoelia, consists chiefly of the two membranes. The 

 transverse ridges upon its ventral aspect iu the frog are indicated by the undulations of 

 the ental line. The roofs of the aula and of the cephalic part of the diacoelia are also mem- 

 branous (aidatela and diatela). In the frog and Menobranchus, although not in the higher 

 Vertebrates, the diacoelian floor is devoid of nervous tissue along the meson, but no special 

 name is given thereto. 



^ 1074. Fig. 113. — Transections of an ideal simple brain at several points. 



The points of transection are indicated by the lines connected with Fig. 110 and by 

 the letters A-H. Of course the continuity of the coelife cannot appear iu these sections, 

 but they combine the distinctive features of the other two in exhibiting at one view the 

 peculiarities of the sides and of the roof and floor. 



(A) Transection of the Myelon. — More accurate representations are given of this in 

 Fig. 99, 100, 109, but this indicates the existence of the canalis centralis, which expands 

 to form the coeliae, and the iieculiar form of the deeply fluted column of cinerea, which 



