408 



Nachdruck verboten. 

 The Segments of the Vertebrate Brain and Head. 



By Alexander Meek. 

 With 5 Figures. 



A study of the development of the brain in the Lesser Black- 

 backed Gull, Larus fuscus L., has led to the following conclusions, 

 which I publish, with a few figures, as a preliminary account. 



Before the closing of the medullary canal and immediately there- 

 after the brain is wide in front and gradually narrows to pass into 

 the spinal cord like region posteriorly. The forward expansion is in- 

 tensified by the early development of the optic vesicles. Even at this 

 period the three primary brain vesicles — prosencephalon, mesencephalon, 

 and rhombencephalon — may be distinguished, and certain important 

 subdivisions of these. 



In the first (Figs. 1 and 2) there are evidences of a subdivision 

 into three parts, a forward region where the neuropore is closing or 

 has closed which I shall call Pro- 

 somere 1 (P,), a middle region 

 from which the optic vesicle ori- 

 ginates and which later gives rise 

 to the infundibulum and the epi- 



Myomeres 1 — 7 



Aud. organ 



tub. post. 

 Chiasma 

 epiphysis 



Fig. 2. 



In all the figures, except figure 2, the hinder limit of the brain and skull is in- 

 dicated by a transverse dotted line. 



Fig. 1. Embryo A. July 3. Slightly oblique view of brain showing segments 

 and somites at an early stage. 



Fig. 2. Sagittal section of an older embryo, '94', showing neuromeres, which are 

 marked as explained in the text. 



physis — Prosomere 2 (Pg), and a third region between the infundibulum 

 or the tuberculum posterius and the next segment — Prosomere 3 (P3). 



