Sorensen, Study of Epiphysis and Roof of Diencephalon. 49 



fishes and less in younger. It is situated directly in the tela 

 choroidea of the third ventricle. Sometimes it is lost entirely 

 in the tela choroidea and can easily pass unperceived. The dif- 

 ference between Ammocoetes and adult Petromyzon consists in 

 this, that in the case of the latter the ganglion, placed under the 

 organ (zirbelpolster), is larger. 



Parapineal organ and Paraphysis. According to the con- 

 cordant statements of authors the paraphysis takes its origin 

 from the unpaired part of the cerebrum, and is distinguished by 

 this, from the parapineal organ which has its origin in the thal- 

 amus. In adult fish one might easily confuse the parapineal 

 organ which is found pushed considerably forward, with the 

 paraphysis. Upon closer examination however, the paraphy- 

 sis can be seen to be removed from the limit which separates 

 the cerebrum from the thalamus. According to this author this 

 limit is found in the place where the tela choroidea passes into 

 the other thicker wall. In the case of adult fishes this place is 

 marked by a sharp fold in the wall. 



Coats of the Organs. The parietal organs are evaginations 

 of the brain and are therefore enveloped with the same coats as 

 the brain. At the point of the secondary reunion with the 

 right habena the coats of the pineal organ are reabsorbed. At 

 the exterior is found a strong shining membrane, the dura 

 mater. Below this we notice the layer of fine filaments belong- 

 ing perhaps to the pia mater. This author did not find any 

 lymphatic cavities around the parietal organs. 



The cornea of the parietal organs. At the surface of the 

 head of Petromyzon planeri we see between the eyes a white 

 region the size of a coin. It is caused by the absence of pig- 

 ment under the corium. This pigment at first formed there a 

 single superior layer visible across the ( epithelial ) layers of 

 the skin, giving to the body a grey color. At this place the skin 

 (epithelium and corium) is completely transparent. In the an- 

 terior part of this place a brilliant body of striking whiteness ap- 

 pears. It is the pineal organ, which is distinguished in its outline 

 by a white pigment. In young Ammocoetes and adult Petromy- 

 zon the little transparent space is found well developed. In ex- 



