118 THE BRAIN OF THE TIGER SALAMANDER 



actual sequence of the changes. In our Amblystoma material the ventral part of this 

 sulcus seems to shift its position and to be transformed directly into the sulcus 

 preopticus, and it was so described ('39a, p. 262). Rudebeck finds in dipnoans, 

 Necturus, and Triturus that the sulcus intraencephalicus anterior passes from the 

 lateral optic recess dorsalward to the posterodorsal hemispheric ventricle and that 

 the definitive sulcus preopticus arises as a secondary outgrowth from this primary 

 groove. Our specimens of Amblystoma have not revealed this secondary origin of 

 the sulcus preopticus but resemble that of the anuran, Pelobates, as described by 

 Rudebeck ('45, p. 53). 



In Amblystoma the posterior intraencephalic sulcus of von Kupffer persists as the 

 sulcus isthmi of the adult. In the 6-mm. Ammocoetes (von KupflFer's fig. 47) it 

 extends transversely from the plica rhombo-mesencephalica to a point in the floor a 

 short distance spinalward of the tuberculum posterius, i.e., to the fovea isthmi, and 

 it is similar in several other species figured. During larval development of x\mblys- 

 toma it and the related external fissura isthmi shift their relative positions (p. 179). 

 As emphasized above, this plane of separation between cerebrum and rhombenceph- 

 alon, whether or not it is marked by a visible sulcus in the adult brain, in all verte- 

 brates is the boundary between the two chief subdivisions of the brain. 



The distinction between these subdivisions is conspicuous in prefunctional and 

 early functional stages. Coghill ('24, Paper IV, p. 97; '31, Paper X, p. 162) reports 

 that at all stages of development of Amblystoma from premotile to swimming the 

 rate of proliferation of cells and differentiation of neuroblasts is rapid in rhomben- 

 cephalon and spinal cord, on the one hand, and in the cerebrum, on the other hand; 

 but "there is a distinct gap between fields of both differentiation and proliferation 

 at the isthmus. Such a gap does not appear at any other level in the brain or cord." 

 Diencephalon and telencephalon appear to be about equally involved in this process, 

 and so do rhombencephalon and cord; but during these stages growth appears to be 

 initiated independently in these two major divisions of the nervous system. 



The experiments of Detwiler ('45, '46), to which reference has already been made 

 (p. 62), show that ablation of the cerebral hemispheres and visual organs of Amblys- 

 toma in prefunctional stages results in no demonstrable change in size or weight of 

 the medulla oblongata. He finds no evidence that the hemispheres or visual organs 

 exert any morphogenic influence upon the medulla oblongata up to a larval age of 

 48 days (33 mm. in length), and these mutilated larvae are capable of performing all 

 the ordinary feeding reactions. 



The most primitive and fundamental patterns of total behavior 

 are organized in the spinal cord and lower medulla oblongata, par- 

 ticularly at the bulbo-spinal junction. Very early these come under 

 the control of the vestibular apparatus (Coghill, '30, p. 638) and 

 midbrain, and this control must be maintained throughout life if the 

 primitive mass movements are to retain their efficiency (Detwiler, 

 '45; '46). The isthmus, interpolated between these regions, seems to 

 be concerned mainly with the organization and control of local re- 

 flexes (partial patterns) of the medulla oblongata under the influence 

 of both ascending and descending systems of correlation fibers. 



For convenience of description the rhombic h^-ain is here arbitrarily 

 divided into four regions, each with characteristic structure and 

 functions: (1) the bulbo-spinal junction, (2) medulla oblongata, (3) 



