326 AWALi^ XEW YORK ACADEMY OF SCI KXCES 



parisoii with tlie embiyos of eight somites makes it evident that these 

 changes are not wholly to be attributed to inequalities of growth, but that 

 a profound remodeling of the cranial extremity of the neuraxis has taken 

 place in addition, as is shown l)y the alteration in the direction of the 

 optic axes, the inclination of the anterior isthmian sulcus and in the 

 notable lengthening of the midbrain taken in connection witli its dimin- 

 ished height. 



Embryos of Ten Somites. — (Plate XXXIT.) The union of the neural 

 folds has progressed. The anterior neuropore (8) extends through the 

 greater part of the length of the optic vesicles into the thalamencephalon 

 (16), the somatic ectoderm being further adherent as far as the mesen- 

 cephalon (15). In this segment of the brain and for a considerable dis- 

 tance caudad the folds have closed and the neural tube has separated 

 from the ectoderm. The roof -plate is concave and depends into the lumen 

 as a longitudinal ridge. Somewhat in front of the middle of the neu- 

 raxis there is a considerable hiatus in the line of closure of the neural 

 folds and behind this are three small areas in which the ectoderm is still 

 adherent, though closure has been completed (11). The fossa rhom- 

 boidalis (21) falls into two portions of about equal length. Cephalad it 

 is narrow and the neural folds are high, approximated and nearly paral- 

 lel. The caudal region is broad and bounded by low folds erected only 

 in their lateral parts; here the neural plate still passes into the somatic 

 ectoderm by a gradual transition. 



The prosencephalon joins the rest of the neuraxis at an acute angle. 

 The anterior isthmian sulcus (22) is horizontal with ventral concavity. 

 The mammillary region (17) is well marked, as is also the tlialamen- 

 cephalon (16) ; the axis of the optic vesicle is horizontal. A posterior 

 isthmian sulcus (23) defines the midbrain caudally. In embryo No. 47G 

 this vesicle is obscurely divided into two segments by a shallow furrow. 

 The hindbrain has three recognizable segments; the first (31) is trian- 

 gular with its base ventral; the second (12) and third (13) are obliquely 

 inclined and defined by shallow, oblique furrows. The quintal anlage (3) 

 is attached in the interval between the second and third segments and 

 the profundus anlage between the second and first (Plate XXXII, Fig. 2, 

 3a). This is the youngest embryo of our series in which the profundus 

 element can definitely be made out. The acoustico-faeial anlage (4) is 

 separated by a short interval from the quintal anlage ; it occupies the 

 furrow which defines the third hindbrain segment caudally and is con- 

 tinuous with the ganglionic crest of the trunk (not shown in the model). 



A second Embryo of ten somites (Xo. 532) corresponds closely to the 

 embryo just described, except for a somewhat greater degree of union of 



