324 ANNALS XEM^ YORK ACADEMY OF SCIENCES 



into one another by siich gradations that it is difficult and in some sec- 

 tions impossible to determine the precise limits of the ganglion. 



Embryos of Eight Somites.— (VMe XXVII, Figs. 2, 3; Plates 

 XXVTII-XXX.) While the nenraxis sliows little if any increase of length 

 in comparison with the preceding embryos, the closure of the neural folds 

 has been initiated. This is first effected' iy the region immediately in 

 front of the quintal anlage (3), where the folds obtain their greatest 

 height (embryo No. 586). In embryo No. 530 there is an additional 

 point of closure immediately caudal to the quintal anlage. 



The optic sulci (1) have increased in depth and the optic vesicles form 

 prominent, ellipsoidal projections with nearly vertical axes. Evidently 

 they correspond to the arched segments of the optic sulci in the younger 

 embryos; the horizontal caudal continuations of these furrows are now 

 reduced. Caudal to the optic vesicle the wall of the neural tube is flat 

 until near the quintal anlage (3), where a moderate dilation is present. 

 This ganglion has now a triangular form and is shorter than in the pre- 

 ceding embryos. It is attached to the dorsum of the medullary plate 

 near its junction with the somatic ectoderm, corresponding to the inter- 

 val between the two points of closure of the neural tube (embryo No. 530) 

 and at the summit of a slight constriction intervening between two mod- 

 erate dilatations. The first of these dilatations (-)/) has already been 

 mentioned; the second (12) corresponds to the oblique descending por- 

 tion of the quintal sulcus, which as a whole in these embryos is under- 

 going reduction. The acoustico-facial ganglion (J/) is small and has a 

 shallow corresponding sulcus. The ganglion occupies the summit of a 

 constriction immediately behind the qumtal dilatation and is followed 

 by a slighter enlargement (13) of the tube, into which its sulcus merges. 

 It is then, in its sulcus and following dilatation, a repetition on a smaller 

 scale of the quintal anlage. The common ganglionic crest extends from 

 the acoustico-facial ganglion, with which it is continuous to the level of 

 the fourth pair of mesodermic segments. In the caudal half of the neu- 

 raxis the neural folds are lower; at first parallel, they diverge in the region 

 of the rhomboid fossa {21 ) . Here the neural plate of each side comprises 

 a horizontal mesial region, and a smaller lateral erected portion, the two 

 meeting at a rounded angle. In the midline there is a vestige of the 

 primitive groove (6'). 



Embryo of Nine Somites. — (Plate XXXI.) The closure of the neural 

 tube is advancing rapidly. It is not, however, effected by a uniform ad- 

 vance in both directions from the region of earliest closure, but on the 

 contrary is incident at several separate points of the neuraxis, as was fore- 

 shadowed in the eight-somite embryo. In addition to the anterior neuro- 



