THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 433 



developments take place. The fold in the brain flooi' behind 

 the thalamencephalon has become still better marked, and 

 over it the brain roof bulges upwards in what will later be the 

 roof of the mid-brain. Posterior to this the brain-roof is 

 distinctly thinner, foreshadowing the membranous condition 

 of the roof of the fourth ventricle. 



In the floor of the thalamencephalon a depression has 

 appeared, separating two elevations. Of these latter the 

 anterior one is the rudiment of the anterior commissure, the 

 posterior of the optic chiasma, while the depression between 

 is the preoptic recess. 



Stage 31 (fig. D). — The fold of the brain floor behind the 

 thalamencephalon has now increased very much in height ; a 

 downfolding of the roof has also appeared, bounding the 

 mid-brain roof behind. 



The elevations in the floor of the thalamencephalon formed 

 by the rudiments of anterior commissure and optic chiasma 

 are now much more strongly marked though still without 

 fibres.^ The pineal rudiment makes its appearance about 

 this time as a small simple diverticulum of the brain roof, 

 and in fact the thalamencephalon has assumed very much its 

 definitive character. 



The anterior wall of the thalamencephalon at about the 

 middle of its height is prolonged upwards and forwards as a 

 cylindrical tube passing between the hemispheres, and ending 

 blindly by a rounded end in front. This must be the para- 

 physis, and it closely resembles the paraphysis of Urodeles." 



Stage 31 + (fig. E). — Fibres have now appeared in the 

 chiasma and anterior commissure. The front wall of the 

 thalamencephalon is thinning out. The roof of mid-brain and 

 thalamencephalon is of practically uniform thickness through- 

 out, and there is no definite distinction between the two. 

 Both the superior and the posterior commissures are well 



' lu regard to tlie region of the tlialaiuenceplialon, I propose to adopt tlie 

 usage of the terms epiphysis, parapliyiis, etc., favoured hy Gaupp (Aiiat. 

 Hefte,. Abth. 2, Bd. vii, S. 229). 



- See Miiiot, ' Anier. Jourii. Atiat.,' vol. i, \i. 'J3. 



