294 ROBERT J. TERRY 



Basal plate and occipital region in smaller embryos 



In van Wijhe preparations of embryos of 10 mm. (fig. 5) 

 there appears in the floor of the occipital region a pair of faintly 

 stained, elongate parachordal plates. These are united anteriorly 

 by a commissure beneath the notochord, but are separated across 

 the midline in the rest of their extent. Each plate is thickened 

 at its caudal end, and is then continued laterally into the occipi- 

 tal arch. The lateral margin of the parachordal plate presents a 

 prominent angle subdividing it into an anterior oblique part oppo- 

 site the mesenchymal cochlear capsule, and a posterior straight 

 part opposite the jugular vein. In the angle between the straight 

 part of the margin and the lateral occipital arch are the three 

 roots of the hypoglossal nerve. Between the parachordal plates 

 and behind their commissure is a vacuity traversed by the 

 notochord. This space is constricted at its middle by opposite 

 projections from the medial edges of the parachordals. The 

 notochord extends forward as far as the hypophysis and appears 

 to lie in a tract of blue-stained tissue which terminates just 

 behind the hypophyseal cartilage. Each lateral occipital arch 

 is slender and cylindrical medially where it joins the parachordal 

 plate, expanded and flat laterally at its free extremity. Where 

 these two parts come together the cartilage is only slightly 

 stained. The lateral arches at this stage lie far removed in a 

 caudal direction from the otic capsules which are represented 

 by parts of the anterior and posterior semicircular canals. 



The atlas has the form of a transverse arch open dorsally. 

 Each lateral extremity of the arch, deeply stained, is in the 

 form of a broad plate presenting two processes, the neural and 

 atlantal. The middle, slender hypochordal part of the arch is 

 only slightly stained. The epistropheus likewise forms an arch, 

 the ventral medial part of which includes the notochord. Here 

 a darkly stained tract on each side is separated from the noto- 

 chord medially and the deeply stained neural arch laterally 

 by very lightly stained zones. The two tracts' near the noto- 

 chord are the beginnings of the centrum of the epistropheus. 

 Just cephalad of the epistropheal centrum and separated from 



