316 JOHN AVARREN 



Fig. 3 is from a similar model of the brain of Laeerta muralis 

 of 2.4 mm. The velum forms a more striking angle in the roof 

 and is now prolonged into a very distinct ridge passing behind 

 the optic stalk to a thickening in the floor of the diencephalon. 

 The paraphysal arch, P. A., is now distinct. It curves forward 

 and upward in front of the velum to form the roof of the telen- 

 cephalon and passes into the upper end of the lamina terminalis, 

 L. T., but there is no sign yet of the paraphysis. Behind the 

 velum is the post velar arch, P. V. A., ending at another low arch 

 or thickening in the roof, E. A., which is the beginning of the epi- 

 physal arch. Both arches therefore develop from the roof of the 

 first diencephalic or parencephalic segment, /. D. Behind the 

 epiphj'sal arch is the last segment of the diencephalic roof, form- 

 ing a distinct arch, S. The lateral ridges bounding this second 

 diencephalic segment (synencephalon), //.Z)., are now more clearly 

 shown than in the preceding stage. We have here a series of 

 primary arches in the roof of the fore brain, each of which is devel- 

 oped from one of the three subdivisions of the fore brain; — the 

 paraphysal arch from the telencephalic, the post velar arch and 

 epiphyseal arch from the first diencephalic, and the intercalated 

 or synencephalic arch from the second diencephalic segment. 



Fig. 4 is a lateral view of fig. 3 and shows clearly the subdivi- 

 sions of the fore brain and the grooves separating them that cor- 

 respond to the internal ridges seen in fig. 3. At the ventro-lateral 

 part of the telencephalon, T, is the optic stalk lying cephalad to the 

 groove V, which corresponds to the velum and separates the tel- 

 encephalon from the parencephalon of first diencephalic segment, 

 I. D. A well defined groove separates the first diencephalic 

 segment from the synencephalon or second diencephalic segment, 

 //. D., which in turn is bounded caudad by another groove sepa- 

 rating it from the mid brain. r^' 



Fig. 5 shows the brain of Laeerta agilis of 2.4 mm. Here we 

 have all the details of the previous stages rather more clearly 

 marked, but the beginning of the pineal organ, E., is now seen 

 as a slight diverticulum growing from the epiphysal arch, E. A., 

 fig. 3. ' ,. 



A 

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