164 



THE GERM LAYER DERIVATIVES 



and the diencephalon. It extends into the diocoel and into the two 

 telocoels, and is therefore tri-radiate. 



The Diencephalon (Thalamencephalon or Between-brain). 

 The third ventricle is divided between the telencephalon and the 

 diencephalon, but the enlarged portion found surrounded by the dien- 

 cephalon is identified as the diocoel. This cavity extends antero-dorsally 

 beneath the epiphysis, antero-ventrally into the optic recess, postero- 

 ventrally into the infundibulum, and laterally into the relatively large 

 optic vesicles. 



The structural derivatives of this diencephalon include the posterior 

 commissure, just anterior to the dorsal limit of the mesencephalon. 

 Anterior to this is the epiphyseal recess, and the dorso-mesial saccular 

 outgrowth known as the epiphysis. This continues to grow forward 

 and becomes separated from the brain as a small knob of cells which 

 remain in the adult as the brow spot. It is presumably homologous to 

 the pineal gland of higher vertebrates. 



Anterior to the epiphysis, in the roof of the diencephalon and be- 

 tween it and the anterior choroid plexus, are the habenular ganglion 

 and commissure. In front of this there later develops a dorsal out- 

 growth know as the paraphysis. In the floor just posterior to the optic 

 recess is the very thick optic chiasma which will contain crossed 

 fibers from the two optic stalks. Posterior to this chiasma is a thin- 

 walled pocket or trough projecting beneath the anterior tip end of the 

 notochord, known as the infundibulum. The cells of the infundibulum 



Rhombencephalon 

 Auditory vesicle 

 Posterior cardnal vein 

 Vitell ne arteries 



Nerve VII 

 Nerve V 

 Mesencepholon 

 Epiphysis 

 Anterior card ' 

 no! vein 



Optic cup 

 Lens 



Caudal vein 



Coudol artery 



Cloaca 

 (Proctodeunn) Anus 



Right vitelline vein 

 Pronephric due' 



Sinus venosus 



Olfactory pit 



-Internal carotid 

 artery 

 Hypophysis 

 Oral evagmation 

 Pharynx 

 Sucker 

 Thyroid 

 Aortic arch 



Reconstruction of the 7 mm. frog larva showing the major organ systems from 

 the right side. (Redrawn and modified after Huettncr.) 



