AMPHIOXUS AND AMMOCCETES. 165 



the visceral branch of the eighth, ninth, or tenth spinal 

 nerve.* 



Stomodoemn, Hypophysis, and Gill-slits. 



It is a common fact that the time and order of forma- 

 tion of corresponding parts differ greatly in the develop- 

 ment of different species. Thus in Ammocoetes, at the 

 stage shown in Fig. 91, the definitive mouth, correspond- 

 ing to the velum in Amphioxus, has not yet formed, but 

 the equivalent of the oral hood is already present in the 

 form of a deep in-pushing of the ectoderm which, at its 

 blind end, is closely applied to the anterior endodermic 

 wall. The mouth will break through later in the middle 

 of the area of contact between ectoderm and endoderm. 



This ectodermic invagination, whose cavity is probably 

 the homologue of the vestibule formed by the oral hood 

 which leads into the mouth in Amphioxus, is known as 

 the stoiitodcEuvt. Immediately in front of the stomodoeum 

 is another ectodermic involution which is in contact with 

 the front of the brain, and is known as the hypophysis or 

 pituitary body}^ 



It will appear later that this is the probable equivalent 

 of the so-called olfactory pit of Amphioxus. 



In the wall of the pharynx of Ammocoetes there are, at 

 this stage, the indications of eight pairs of gill-slits. They 

 have not yet, however, broken through to the exterior, but 

 consist of a succession of hollow outgrowths of the phar- 

 ynx stretching towards the ectoderm with which they will 

 eventually fuse (Fig. 92 A, B, C). 



In the case, however, of the first pair of gill-pouches, 



* It is not impossible that many of the rami viscerales may send up branches 

 to the branchial plexus, as was indeed described by Rohon. In this case. 

 Van Wijhe's nerve would be of the nature of a collector. 



