404 



il, but, I should say from my observations, that, more frequently, it 

 is not so widely expanded. It always settles down, in Squalus 

 acanthias, to occupy the position first indicated, and, subsequently, 

 it is shifted backwards- In fig. 7 the topography of the head region 



Fig. 7. Head of embryo of Squalus acanthias after complete closure of the neural 

 groove, but before formation of the auditory vesicle. Reference marks as in previous 

 Fig. Compare the position of the first three neural segments with the same in Figs. 3 

 and 6. 



is similar to what it is in fig. 6: — in front is the primary optic 

 vesicle, and behind it, looking like a second eye, is the first accessory 

 vesicle, the second accessory vesicle no longer exists as a distinguish- 

 able organ — in its place has appeared the beginning of the mid- 

 brain (mb). The position of the mandibular cavity (He), is clearly 

 marked and we may also distinguish the position of the future first 

 visceral cleft; faintly outlined are the boundaries of two branchial 

 arches. 



There has evidently been some shifting of parts since the previous 

 stage. There are now only two segments in front of the eye, instead 

 of three, as in the former stage. Metamere number three, occupies 

 the position between the primary and the first accessory optic vesicle, 

 likewise, no. four, is now in front of the latter structure etc. The 

 segment marked 8 serves as an important landmark in all subsequent 

 changes that affect the segments. It is seated above a depressed 

 region in which the first visceral cleft subsequently appears, and, 

 during all the time the segments are distinguishable from the out- 

 side, it has no nerve root. 



Only a few words of description will be needed to enable us to 



