NOTES ON ELASMOBRANCH DEVELOPMENT. 253 



of the origin of the first somite. It is true that at the time 

 of the formation of the medullary plate the notochord stops 

 some little distance short of the front end of the body, and 

 there is a portion of the gut in front of it ; but this is only a 

 temporary state of affairs, and is due to the fact that the front 

 end of the notochord, which is developed from behind for- 

 wards, is not yet formed : moreover the solid mass of endo- 

 derm referred to by Kastschenko is present at the front end of 

 the gut even at this stage. When the notochord has acquired 

 its furthest anterior extension in Scy Ilium, just before 

 Stage G, it terminates in a solid mass of cells, which is con- 

 tinuous also with the front end of the gut. The notochord 

 has hitherto during the whole of its growth been continuous 

 in front with the endoderm, and its condition at the period 

 referred to is merely a persistence of that continuity. Wyhe's 

 account of the anterior end of the notochord appears to me to 

 be quite correct. 



When the notochord has acquired its utmost anterior 

 extension there is no portion of the gut in front of it, but 

 merely this solid mass of cells, with which both it and the gut, 

 and afterwards the ectoderm of the buccal slit and pituitary 

 body, are continuous, and which underlies the very front end 

 of the medullary tube. If this mass of cells be regarded as 

 partly consisting of the anterior end of the notochord still un- 

 differentiated, it may be said that the notochord reaches in 

 Scy Ilium, at any rate, to the very front end of the neural 

 tube; in other words, that Scy Ilium at this stage is truly 

 cephalochordate in the sense that Amphioxus is cephalo- 

 chordate. 



The solid mass of cells in which the notochord and gut 

 terminate becomes in Scyllium and Pristiurus very early, 

 before Stage G, connected with the ventral ectoderm, Wyhe, 

 who connects this fusion with the formation of the mouth, 

 puts it down as taking place later in Stage H ; but I can posi- 

 tively assert that in Scyllium and Pristiurus it is present 

 before Stage G — before any trace of the cranial flexure has 

 appeared. 



