SCHULTE AXl) TILXEY, XEURAXIS IN THE DOMESTIC CAT 333 



(4) which is not accompanied by any definite local protrusion of the 

 wall but simply grooves the medullary fold entally near the neurosomatic 

 junction, becoming broad and shallow and disappearing before the floor- 

 plate is reached. This furrow corresponds to the acoustico-facial anlage 

 in older emln-yos. We would note further that on the right side there 

 is a small pit situated midway between the optic and quintal sulci having 

 the same characters as the latter, except that it is unaccompanied by an 

 external protuberance and is present only in three sections of 13.3 micra 

 each. This had no homologue on the left side. It is possible that this 

 minute furrow represents the profundus anlage, although clear evidence 

 of its presence is not found in our embryos until the stage of twelve 

 somites is reached. A similar conformation was observed farther caudad 

 on the left side in the region behind the last somite; it extended over 

 three sections. Caudad the medullary folds divaricate and liecome lower, 

 eventually disappearing in the region of the primitive groove. In con- 

 trast to the cranial extremity, it is to be noted that here the first portion 

 elevated is the lateral and not the basal zone of the medullary plate. 

 This is characteristic of this region in later stages as well. It is ap- 

 parent, therefore, that, both craniad and caudad, the elevation of the 

 neural folds is accomplished in two phases but that the order of events 

 is reversed at the two ends of the embryo. Craniad the basal region 

 first becomes vertical, then the lateral, while caudad the converse is true. 

 This is the only evidence we have been able to find of a morphologic 

 difference between the basal and alar plates, for the sulcus limitans is a 

 late formation, if it is present at all in young embryos of the cat. 



Embryos of Seven Somites. — The neural folds are separate in their 

 entire length and in general show but little advance in comparison with 

 the embryo of four somites. The optic sulci are strongly arched and the 

 prominence of the primitive optic vesicles is slightly increased. The 

 tubercle of the floor intervening between the two optic sulci forms a well 

 defined cranial limit to the floor of the neural tube, and blending with 

 the wall in front of the optic sulci forms the ventral lip of the neuropore. 

 The quintal and acoustico-facial sulci consist of horizontal and obliquely 

 descending segments, the latter in each case becoming broader and shal- 

 lower as they approach the floor-plate. On the ectal surface of the neural 

 plate faintly marked oldique elevations correspond to the oblique portions 

 of these sulci. The quintal anlage forms an elongated ridge, extending a 

 little farther craniad than the horizontal segment of the quintal sulcus. 

 Its extremities now project free of the medullary plate, no longer fusing 

 with it as in the embryo of four somites. This anlage enters into inti- 

 mate relations with the mesenchyme of the head, the two tissues passing 



