SCHULTE AXD TILNEY, yEURAXIS IN THE DOMESTIC CAT 343 



are no signs of compression in the mesencephalon bnt the slight bulge 

 of the mammillary region is, perhaps, the result of flexure. 



In embryos of ten to twelve somites (Plates XXXII and XXXIII), 

 the mesencephalon acquires a triangular profile, demarcated from the 

 hindbrain by a shallow constriction which gradually becomes pronounced 

 and has a vertical direction (the posterior isthmian sulcus 23). The 

 anterior isthmian sulcus (22) is horizontal and forms a sharp boundary 

 against the mammillary region but becomes shallow cephalad near the 

 pole of the optic vesicle. A slight transverse depression of the roof 

 separates the midbrain from the thalamencephalon (16). The walls are 

 convex and entally show nothing which can be taken for the sulcus 

 limitans of His. A'entrad the isthmian sulci converge to the angle made 

 by the forebrain with the floor of the neuraxis. 



In the thirteen and fourteen somite embr\^os (Plate XXXIV), the 

 mesencephalon is gradually bent ventrad and comes to form the most 

 cephalic point of the brain. This bend is associated with clear evidences 

 of compression in the floor of the hindbrain. In the sixteen-somite 

 embryo (Plate XXXY), the mesencephalon has been carried slightly 

 beyond the crown of the cephalic arch and conjointly with the prosen- 

 cephalon (19) makes a right angle with the hindbrain. The isthmian 

 sulci now converge at an acute angle and the midbrain reaches the ventral 

 margin only by its pointed extremity. At) this stage, a nuchal bend is 

 well defined and the efl'ects of compression upon the mesencephalon are 

 at a maximum. From this period to that of twenty-one somites, the 

 midbrain lengthens in its dorsal segment and chiefly in a cephalic direc- 

 tion, as is shown by the alteration in the angle at which the anterior 

 isthmian furrow meets the floor as well as by the increased flexure of 

 the forebrain (Plates XXXVI-XXXVIII) . AVe have described the mid- 

 brain as a single segment without subdivision into neuromeres, for, 

 though we have searched for evidence of a constriction. Ave have been 

 able to find none in any of our embryos save that of ten somites, X"o. 

 476, and here with less certainty than could be wished. A faint con- 

 cavity was present between the isthmian furrows and inclined so that 

 its continuation would have bisected their angle and divided the mid- 

 brain into a slightly larger cephalic and smaller caudal portion. It was 

 confined to the dorsal portion of the neural plate without, however, 

 causing a depression in the roof. The brain in this embryo was some- 

 what spirally twisted to the right and the depression in question was 

 not quite sjTnmetrical on the two sides. For this reason and because 

 in all our other embryos the midbrain attains its greatest width precisely 

 in the region where this exceptional furrow appears, we are inclined to 

 attribute its presence to the unusual twist of the head. 



