DEVELOPMENT OF THE NERVOUS SYSTEM. 



6: 



anterior head cavities mentioned above. This most anterior 

 mass of entoderm is present in amphibia also (P'ig. 36), although 

 the anterior head cavities are not separately developed. In 

 this is seen the vestige of a communication between the hypoph- 

 ysis and the archenteron. In many vertebrates an outgrowth 

 of entoderm from this region, kno^^^l as Seesel's sac, enters with 

 the ectodermal pit into the formation of the adult hypophysis. 

 In cyclostomes the ectodermal pit becomes deep and large, and in 

 Bdellostoma (Kupffer) has an open communication with the arch- 

 enteron at an early stage. This communication is afterward oblit- 



Epiphysis 



Optic chiasma 



Mouth 



Fig. 36. — A median sagittal section of the head of an embryo of Amblystoma 

 punctatum, to show the relations of epiphysis, velum, paraphysis, hypophysis, 

 Seesel's sac, etc. 



erated but the hypophysis remains throughout life as a large sac 

 open to the exterior and extending beneath the brain. The communi- 

 cation with the archenteron has been described also for the sturgeon. 

 Also in Amphioxus, where the permanent mouth is formed from 

 a gill sHt, a canal which opens externally farther forward communi- 

 cates with the archenteron at an early stage of development (Leg- 

 ros). The relations of mouth and hypophysis in Amphioxus and 

 Petromyzon are represented diagrammatically in the accompany- 

 ing figure from Legros (Fig. 37). The ancient vertebrate mouth, 

 or paleostoma, is to be thought of as lying very near the anterior 

 end of the animal, just beneath the region of the olfactory organs 

 which alone extend farther forward. In the sides of the canal 



