THE INTEEBEAIN OE THALAMENCEPHALON". 



129 



It is known that in the sturgeon a passage leads from the exterior into 

 this preoral gut: i.e., the fundament of a separate mouth is established over 

 the permanent mouth. This whole structure — the preoral cavity and the 

 preoral gut, into which it leads — lecomes the hypophysis. According to 

 KupfEer, the evagination from the oral cavity of craniate vertebrates — the 

 hypophysis — is a vestige of this old preoral cavity. 



In lower vertebrates dorsal to the infu.ndibulum the posterior wall of 

 the infundibulum is evaginated into a long, narrow epithelial tube whose 

 walls are thrown into numerous folds through the pressure of numerous 

 blood-vessels. The structure is called the Saccus vasculosus (see Figs. 59 

 and 77). When the posterior end of the ventral wall of the interbrain joins 

 to the base of the midbrain one always finds a farther small evagination: 



Fig. 78. — Horizontal section through the hypophysis of the ray: Baja clavata. 



the Recessus mammillaris. In selachia it contains large ridges and nodules 

 of epithelium, and forms a richly vascular structure evidently functional. 



Now that the dorsal and ventral portion of the interbrain have been 

 described, we may turn to the consideration of the lateral portions. Close 

 to the epithelial roof there lie the Ganglia habenulce, one on either side, the 

 distinctive ganglia of the Epithalamus (Fig. 75). In many of the lower 

 vertebrates there is a difference in the size of the two ganglia; otherwise, 

 however, the Ggl. habenuls offer a good example of a markedly constant 

 brain-structure, varying neither through progression nor retrogression. 

 From Petromyzon to the mammals one always finds them on either side and 

 a little to the front of the epiphyseal process. They consist of two bodies, — 

 a lateral and a median, — and are always separated from the epiphyseal sac 

 posteriorly by the Commissura hahenularis (Figs. 76, 79, 86). In am- 



