522 LOCV. [Vol. XI. 



about by changes in the medullary folds. The first three 

 metameres are, at this stage, in front of the eye, the fourth, 

 fifth, and part of the sixth, in front of the accessory optic and 

 mid-brain vesicle. The following five segments (seven to 

 eleven) occupy the reflected part of the neural fold. The 

 eleventh, as has been indicated, lies in front of the vagus 

 nerve. 



The next stage to be considered (PI. XXVII, Fig. 32) is just 

 after the closure of the neural groove, which is, however, still 

 open in front by a small neuropore. The original segments 

 are still visible throughout the head region ; those in the fore 

 and mid-brains still show from the outside. The surface- 

 markings on the head are similar to those in Fig. 31. In 

 front is the conspicuous optic vesicle, and behind it is another 

 similar rounded eminence, the mid-brain vesicle. Further 

 back there is an area, circular in outline, resembling the mid- 

 brain, except it is not so prominent. It is, however, merely a 

 pad of mesoderm applied to the walls of the cerebellum. There 

 is another surface indication of considerable interest, viz., a 

 line of mesoblast running over the branchial region ; it con- 

 nects in front with the mandibular cavity and behind with the 

 body protovertebrae. 



Fig. 33 shows a slightly older stage than Fig. 32. The 

 neural groove is completely closed. The extreme anterior end 

 of the gastrular cavity has been obliterated. The cranial 

 flexure is quite marked. This is the last stage in which the 

 metameres are visible throughout the length of the embryo ; 

 those in the fore- and mid-brain have become indistinguishable 

 in surface views ; they are, however, still to be detected in 

 longitudinal sections. We possess now a particular advantage 

 in dealing with these segments, because anatomical landmarks 

 of the head regions have become established, and these enable 

 us to say with definiteness what are the relations of the 

 segments to the rest of the head. This is just prior to the 

 appearance of the auditory vesicle ; when first established its 

 center occupies the space of the segment marked 10. Some- 

 times, in its earliest stages, the circular area spreads over the 

 space of the three segments marked 9, 10, and 11, but I 



