THE VERTEBRATE HEAD. 587 



EXPLANATION OF PLATE XXVIL 



All the figures are of Squalus acanthias ; they are drawn with the aid of the 

 camera, and are all X 45 diameters. 



Fig. 25. Young embryo intermediate between Balfour's stages ^and C. The 

 embryo so far as formed is divided into eight pairs of metameres and these are 

 continued without break, or any change in character, into the halves of the 

 embryonic rim. The eleventh metamere which, in later stages, lies in front of 

 the vagus nerve is now on either side the third one from the axial embryo. 



Fig. 26. Somewhat older embryo, showing the change in form of the head 

 region. The axial embryo now includes about fifteen pairs of metameres. 



Fig. 27. Slightly older than the preceding ; there are about eighteen pairs of 

 metameres in the axial part of the embryo and, as in the former instances, are 

 continued into the embryonic rim. Two longitudinal marginal furrows have 

 appeared, separating two marginal bands from the rest of the embryo. Along 

 the line of these furrows are seen four depressions that mark the very beginning 

 of segmental sense-organs. 



Fig. 28. View from the upper side of the same embryo illustrated in Fig. 29. 

 The cephalic plate is now clearly marked off from the more slender trunk region. 

 The depressions for the optic vesicles (op)) have made their appearance. 



Fig. 29. View of the same embryo from the ventral aspect. The yolk has 

 been completely removed, and we get a view directly into the gastrular cavity. 

 There are eleven pairs of metameres in the broad part of the cephalic plate. The 

 neural folds are ventrally curved. The outlines of the figure and the neural seg- 

 ments are too symmetrical. 



Fig. 30. Older embryo with neural folds lying in the horizontal plane. The 

 broad cephalic plate is in marked contrast with the slender trunk. 



Fig. 31. Embryo in which the neural folds have nearly attained the vertical 

 plane. The neural groove is still open. The optic vesicle {op) and the combined 

 vesicle of mid-bram and accessory optic (7tib. -f- A. op.) vesicle show on the sides 

 of the head. There is also the beginning of mandibular head cavity (H. C.) 

 and the branchial pouch. The original metameric divisions are still very plain. 



Fig. 32. Embryo just after the closure of the neural groove in the anterior 

 end. The posterior part of the neural canal is not completely closed. Note the 

 metameric divisions indicated by numbers /, 2, j, etc. 



Fig. 33. Embryo after complete closure of the neural groove and before the 

 appearance of the ear vesicle. 



Fig. 34. Embryo after the differentiation of the ear saucer. The five anterior 

 metameric divisions are no longer distinguishable, those of the hind-brain are 

 prominent and are approximated in the middle plane. One gill-cleft has broken 

 through. The nasal pit has started. 



Fig. 35. Slightly older embryo showing several characteristic changes. The 

 line of neural segments are being forced apart by lateral growth of the roof of the 

 hind-brain. The fifth nerve is plainly visible. Over the gill-clefts runs a con- 

 tinuous nodulated thickening containing the branchial sense-organs and the rudi- 

 ment of the lateral line. 



