360 COMPARATIVE ZOOLOGY OF CHORDATES 



in the primitive selachian Heptanchus (both of which have 8 

 visceral slits, although Petromyzon loses one), but Amphioxus 

 in its development passes through a stage (the so-called 

 " critical stage ") when it has 8 pairs of symmetrically arranged 

 gill-slits. 



The relations and destinies of the three prootic somites are 

 constant in all vertebrates above the Cyclostomes (in which the 

 eyes are degenerate), and they may now be considered. 



In the first place, it is interesting to note that the ist or 

 premandibular somites correspond to the anterior head- 

 cavities or anterior gut-diverticula of Amphioxus, and that 

 the front ends of all chordates correspond. Just as in 

 Amphioxus the anterior head- cavity (of the left side) opens into 

 an ectodermal pit (the preoral pit), so in Selachians (Torpedo) 

 the premandibular somites open into an ectodermal inpushing 

 (the hypophysis), and this connexion between premandibular 

 somites and hypophysis also occurs in some reptiles and birds. 

 The hypophysis is therefore probably homologous with the 

 preoral pit of Amphioxus. This connexion between a meso- 

 dermal pouch and the ectoderm is similar to that which occurs 

 in Balanoglossus and the larvas of Echinoderms, forming the 

 co-called " water-pores " and " proboscis-pores." (It may 

 be mentioned that the so-called " anterior head-cavities " of 

 some Selachians are merely parts of the premandibular 

 somites, and have no segmental value.) 



The morphological anterior end of the body in Craniates is 

 a point near the middle of the mesodermal strand connecting 

 the premandibular somites with one another. Just behind 

 this point is the front end of the notochord, and the preoral 

 gut ; just in front of it the hypophysis grows in from the 

 superficial ectoderm, and just above it is the floor of the fore- 

 brain near the optic chiasma and the point of closure of the 

 neuropore. This morphologically anterior point of the animal 

 is represented in many skulls near the dorsum sellae, which 

 lies immediately behind the pituitary body. That part of the 

 head which lies in front of this is the result of secondary 

 forward growth. 



In the conversion of the prootic somites into the eye-muscles 



