BASIC DEVELOPMENTAL FEATURES 



813 



PROSENCEPHALON —1 \ 



( FOREBHilN ) L TELEN 



Fig. 354. Early development of the brain in the chick and teleost fish showing the 

 tendency to form neural segments or neiiromercs. (All figures redrawn from Hill, 1900, 

 Zool. Jahrbiicher, abt. Anat. u. Ontogenie 13.) (A) Dorsal view of developing brain of 

 chick embryo of 4 pairs of somites. (B) Dorsal view of primitive brain or encephalon 

 of chick embryo of 7 pairs of somites. (C) Dorsal view of brain of chick embryo 

 with 11 pairs of somites. (D) Dorsal view of developing brain of chick embryo with 

 14 pairs of somites. (E) Lateral view of brain of chick embryo about 75 to 80 hours 

 of incubation. In the foregoing illustrations, observe that the neuromeres gradually fuse 

 to form parts of primitive five-part brain shown in E. (F) Brain, lateral view, Salmo 

 fario, 33 somites, 22 days old. Segments 1-3 represent the prosencephalon, 4 and 5 the 

 mesencephalon, 6 the anterior part of the rhombencephalon, and 7-11 to the posterior 

 region of the rhombencephalon. Observe that the cephalic flexure is present slightly at 

 this time. A little later in the 36 day embryo it is more pronounced. 



(3) The acoustic placodes, two in number, taking their origin from the 

 dorso-lateral portion of the epidermal tube overlying the middle por- 

 tion of the hindbrain. 



In water-dwelling vertebrates, other placodes arise in the head region as- 

 sociated with the lateral-line system. The lateral line placodes probably repre- 

 sent an extension of the acoustic placodal system in lower vertebrates. Hence, 

 the general term acoustico-lateral or neuromast system (see Goodrich, '30, 

 p. 732) may be applied to this general system of sensory structures. 



(4) Taste-bud placodes. The taste buds are distributed variously in dif- 

 ferent vertebrate species. In man, cat and in other mammals they are 

 located on the tongue, particularly its posterior part (fig. 285E) on the 



