SCHVLTE AND TJLNEY, NEVRAXI8 IN THE DOMESTIC CAT 337 



that this is in the interests of a hypothetical inrolling either here or in 

 the optic vesicles. 



At nine somites the neuropore is shorter, b_y reason of closure at both 

 its ends but chiefly caudad. Its extremities now lie in the same vertical 

 plane. The quintal hiatus is closed at two intermediate points, present- 

 ing three small orifices (Plate XXXI, Fig. 1). Caudad to it a short 

 segment of the tube has closed. A small hiatus is present in the region 

 of tlie acoustico-facial ganglion. This embryo, therefore, gives addi- 

 tional evidence of the retardation of closure in regions of large ganglia. 



Further irregularities are shown in Plate XXXII, Fig. 1, and Plate 

 ,XXXIII, Fig. 1, in the trunk region, and in Plate XXXIII, Fig. 2, 

 and Plates XXXIV-XXXVI in the anterior neuropore. As this latter 

 closes ventrally as well as caudally, and irregularly in the intermediate 

 portion, it seems difficult to consider that its region of latest obliteration 

 has any fundamental morphological importance or can at all properly 

 be used to determine the cephalic extremity of the neuraxis, which is 

 certainly deflected ventrad. Further, if the wall of the neural tube is 

 divided into basal and alar plates, its cephalic pole ought to be the most 

 cephalic point in their line of union, i. e., in the terminology of His, 

 the cephalic extremity of the sulcus limitans, when this sulcus can be 

 recognized. It would seem, therefore, that wherever this point is local- 

 ized it cannot be situated in the raphe which throughout its length is 

 assumed to be a suture between the summits of the alar plates, or, if 

 our interpretation prove correct, in the forebrain, between retained gang- 

 lionic zones. To accept the last point of attachment of the ectoderm 

 marked by the recessus neuroporicus as the extremity of the axis, implies 

 that the raphe below this point is a suture between the basal plates, 

 although it has never been shown that they were primitively cleft; 

 further it would seem the necessary consequence of the acceptance of 

 this landmark (recessus neuroporicus) that the mammillary and in- 

 fundibular regions and the ventral half of the optic vesicles themselves 

 were derived from the basal lamins. To accept the recessus neuroporicus 

 as the ontogenetic pole of the brain seems, therefore, to disregard the 

 ventral deflection of the neuraxis and' the composition of its wall of 

 basal and alar plates. In His's three months' embryo (Xo. 7 of Ziegler's 

 series of models) the sulcus limitans passing forw-ard in the midbrain 

 is continuous with a furrow which arches ventrad and reaches the mid- 

 line immediately in front of the oculomotor nucleus. This, we believe, 

 is actually the sulcus limitans demarcating the alar and basal laminte 

 and reaching the midline where the latter ceases to give evidence of its 

 existence, i. e.. immediately in front of the oculomotor nucleus, the most 



