100 JOHN WARREN 



having only a rudiment of a paraphysal arch in the roof of the 

 telencephalon, with absolutely no sign of a paraphysis, which 

 must be morphologically of telencephalic origin. The important 

 features in the development of the pineal region in these em- 

 bryos are the almost total suppression of the paraphysal arch, 

 the much reduced velum and the luxuriant development of the 

 plexus. An excellent account of the development of the fore- 

 brain in the cat is given by Tilney (23) and figures of embryos 

 up to 70 mm. are there shown. 



CONCLUSIONS 



1. The primary arches in the roof of the telencephalon and 

 diencephalon are all seen in early stages of mammalian embryos, 

 and are all present at the same time. The marked extent of 

 the synencephalic arch or pars intercalaris and its early appear- 

 ance is a very striking feature in Mammalia. 



2. The three main subdivisions of the forebrain, the telen- 

 cephalic and the two diencephalic segments or subdivisions, 

 can be followed in all the Mammalia referred to here. 



3. The paraphysis is found definitely developed in sheep 

 embryos only. In all other Mammalia except man it is repre- 

 sented merely by the paraphysal arch either with or without a 

 transverse fold which passes across the whole width of the telen- 

 cephalic roof. In human embryos it can be found in occasional 

 cases as a rudimentary and very inconstant structure. 



4. In all the earlier stages of mammalian embryos the velum 

 is fairly well developed. In later stages it becomes reduced, 

 especially towards the mid-line, to a mere angle and is more or 

 less absorbed in the folds of the diencephalic plexus. 



5. The postvelar arch is always well developed. Later the 

 diencephalic roof plate becomes incorporated mth the tufts of 

 the diencephalic plexus which occupies the greater part of its 

 extent. There is in most cases a well marked suprapineal recess 

 extending upward and backward over the supracommissure. 



6. The epiphysis of ungulates and carnivora even in the oldest 

 embryos studied is very small possessing thick walls and a small 

 lumen. It is almost buried between the two commissures and 



