520 LOCV. [Vol. XI. 



folds have unrolled from their ventrally curved position and are 

 in the process of growing upwards. Those of the head are at 

 this stage nearly in the horizontal plane (cut 6). The outer 

 margins of the folds are plainly divided into segments, and the 

 segmentation extends backwards, also, into the trunk region, but 

 not so clearly defined. The optic vesicles are clearly seen on 

 the head-plate, but the accessory optic vesicles (see p. 57) have 

 not yet made their appearance. Beginning at the front end 

 and counting backwards eleven segments on either side, we 

 come to the point where the broadly expanded cephalic plate 



««V»..^*.^'»'•■ 









Cut 6. — Six transverse sections of an embiyo older than the preceding one. x about 30 diameters. 

 The numbers below the sections refer to their position in the series. The neural folds are 

 growing upwards and in section 24 have reached the horizontal plane. The depressions in 

 Sections 3 and 12 are the optic vesicles. The embryo from which the sections are made is 

 shown on PI. XXVI, Fig. 7. 



passes into the narrower neck and trunk. This, as before 

 indicated, is the point of future origin of the vagus nerve. It 

 seems to me to be a natural line of division which may be of 

 service in determining the limits of the embryonic head. The 

 question will be returned to on p. 543. 



It should be borne in mind that all the stages so far de- 

 scribed are very young ; the earliest ones are before the forma- 

 tion of the embryonic medullary folds, and the oldest one is 

 just when the medullary folds arc arching upwards to form, for 

 the first time, a medullary groove. The mesenchymic somites 

 have, in the interim, appeared in the trunk region, and have 



