323^ ANXAL8 NEW YORK ACADEMY OF SCIEXCES 



deeper and tlie medidlarv folds have partially erected themselves over 

 a greater longitudinal distance. Craniad also a faintly marked furrow 

 is present in the region where the medullary plate becomes thinner as 

 the somatic ectoderm is approached. In addition, the lateral region of 

 the plate presents slight undulations of the surface. 



Embryo of Three Somites. — (Plate X-XTIT, Fig. 3.) The neuraxis 

 has the same general configuration as in the preceding embryos. The 

 boundary between the medullary plate (-5) and the somatic ectoderm 

 (7) is still effected by a gradual transition (~5). Craniad there is a 

 faint furrow as in the embn-o of two somites, and in this region also 

 the margin is marked by faint undulations. 



Embryo of Four Somites.— {FMe XXYII, Fig. 1; Plates XXI \"- 

 XXVI.) The neuraxis has not lengthened appreciably as compared with 

 the embryos of three and of two somites. In its cranial half, however, 

 the neural folds (5) have fully erected themselves and an abrupt bound- 

 ary has been established between them and the somatic ectoderm (7). 

 Caudad this junction is still effected by a gradual transition; there is 

 no furrow. The floor plate terminates craniad in a distinct thickening 

 and elevation (2) which intervenes between the ventral extremities of 

 the optic sulci (1). In front of them it is continuous with the ectal 

 margins of the primitive optic vesicles. We have designated this eleva- 

 tion the tubercle of the floor. The optic sulci begin on each side of the 

 tubercle of the floor where they are continuous with the angle that defines 

 the floor-plate from the parieties. They describe an arc with the con- 

 cavity caudad and approximately parallel to the margin of the neural 

 plate, but approaching the neurosomatic junction and becoming fainter 

 as they are followed caudad. Corresponding to their arched segment 

 there is a marked thickening and external prominence of the wall of the 

 neural tube. 



At a short distance caudad to the optic sulci there are a pair of similar 

 though shallower furrows, close to the neurosomatic junction. Their 

 course is at first horizontal ; their caudal portions turn ventrad and 

 approach but do not reach the floor-plate. Corresponding to their hori- 

 zontal portion, there is a ridge-like projection between the summit of 

 the neural plate and the somatic ectoderm. This ridge is the quintal 

 anlage (3). That it is more intimately related to the neural fold (5) 

 than to the somatic ectoderm (7) is sho^Mi both by the fact that the 

 neurosomatic junction is dorsal to it and by the fact that it blends at 

 both of its extremities with the thickened dorsum of the neural plate. 

 It is in no sense an element intermediate between neural plate and 

 ectoderm. Just behind the qtiintal anlage is another, smaller furrow 



