344 ANNALS NEW YORK ACADEMY OF SCIENCES 



Rhombencephalon 



The large ganglia of the quintus, acoustico-facialis and profundus, 

 which develop in the order named, are associated in their inception with 

 sulci. These, hegiuning near the neurosomatic junction, and at first 

 parallel to it, eventually turn ventrad pursuing an oblique course across 

 the medullary fold. Each furrow is thus x;omposed of a cephalic seg- 

 ment, intimately concerned in the formation of the ganglia and a caudal 

 portion which, becoming broad and shallow, occasions a dilatation of 

 the neuraxis. As the tube closes three oljlique segments are formed. 

 In the embryo of ten somites (Plate XXXII, Fig. 2), where we were 

 first able to recognize the profundus anlage distinctly, four segments 

 were present. The first (31) is triangular in form with base ventrad, 

 extending from the posterior-isthmian sulcus to the oblique furrow at 

 the summit of which is the profundus ganglion (Sa). This element has 

 no ganglion associated with it developmentally but owes its demarcation 

 to the establishment of a caudal boundary of the midbrain in the poste- 

 rior isthmian sulcus. It is followed by three oblique ganglionic segments 

 which are the expression of the oblique caudal portions of the ganglionic 

 sulci in the interior of tube. Externally three oblique intersegmental 

 constrictions are present, so that the third ganglionic segment lacks a 

 caudal boundarj' and merges into the relief of the myelencephalon. The 

 ganglia are situated at the summits of these furrows and are accordingly 

 intersegmental in their points of attacliment, as was first pointed out by 

 Miss Platt.i" 



This configuration is retained to the stage of thirteen somites. In 

 these embryos, and very vaguely in the more advanced embryos of twelve 

 somites, the surface of the neural tube behind the ganglionic segments 

 becomes marked by alternating constrictions and dilatations. These, in 

 marked contrast to the ganglionic segments, are vertical in position and 

 correspond to the mesodermic somites which abut upon the neuraxis in 

 the intervals between the dilatations. Tliese vertical segments are un- 

 doubtedly the myelomeres of ]\[cClure^^ and are widely different struct- 

 ures from the oblique ganglionic segments of more cephalic position. 

 Primitively the two series are separated by a consideral^le interval which 

 is not effaced until the stage of fourteen somites, the most cephalic 

 myelomeres being relatively late in apj^earance. Their retardation, we 

 believe, is due to the small size of the mesodciinic somites in tliis region. 



i" .T. B. Platt : "A f'ontiilmiion to iln' Minplioluuy of the Vortel)rate Ilpiui bas«ed on a 

 Study of Acanthias Vulsaris." .Tour. Morpli., Vol. V. 1801. 



" C. F. W. McCLrRE : "Tlie Primitive Sesmeiitation of the Vertebrate Brain." Zool. 

 Anz. .lahrg.. Vol. .XII. ISSO. 



