72 CLASS PISCES. 



which probably belongs to the mesencephalon, is just behind the 

 attachment of the pineal stalk. 



The paraphysis is the recess in the roof of the cerebrum caused 

 by and just in front of the velum transversum. By some mor- 

 phologists it is regarded as a special glandular organ, the 

 secretion of which passes into the ventricle. It is not always 

 present as a distinct structure. 



The pituitary body or hypophysis develops as an evagination 

 of the front part of the buccal cavity. It is indeed the anterior 

 part of this cavity. In Elasmobranchs the original buccal slit 

 for the vertebrate mouth perforation has at first the form of a 

 longitudinally extended slit (Fig. 39) is continued into it. 

 It is applied to the infundibulum and eventually becomes cut 

 off from the mouth, except in Polypterus and Calamoichthys in 

 which the buccal opening is retained throughout life. At the 

 point in the embryo where the pituitary rudiment meets the 

 infundibulum there is a close approximation and partial fusion 

 of three other organs, viz., the front end of the gut, the anterior 

 end of the notochord, and the median part of the premandibular 

 somite (preoral coelom). The lobi inferior es and saccus vas- 

 culosus are parts of the infundibulum, very generally present 

 in fishes. The former are lateral diverticula or thickenings of 

 the infundibulum ; while the saccus vasculosus, or infundibular 

 gland, is a glandular dilation of its end, where it is in contact 

 with the pituitary body. 



With regard to the cranial nerves, it ought to be noticed that 

 they all arise from the walls of the mid- and hind-brain, except 

 the olfactory and optic nerves. These come off from the pre- 

 notochordal part of the fore-brain, and it is doubtless to them 

 that the fore-brain owes its relatively enormous development in 

 all Vertebrata. The other cranial nerves, from the third nerve 

 onwards, probably all belong to the series of nerves which is 

 continued along the spinal cord as the spinal nerves. Indications 

 of this are shown by a careful study of the early stages of their 

 growth, particularly in Elasmobranch embryos, in which they 

 appear to be associated with the cephalic segments of the coelom. 



These segments, for a knowledge of which we are indebted to Balfour* 

 and to the later researches of Van Wijhe.f are as follows : The first cranial 



* A Monograph of the Development of Elasmobranch Fishes, London, 1878. 



f " Ueber die Mesodermsegmente und die Entwickelung der Nerven des 



Solachierkopfes," Verhandl. der k.Acad. d. Wissensch. zu Amsterdam, 1882. 



