PINEAL REGION IN MAMMALIA 83 



No trace of the velar commissure described in Reptilia by 

 •Elliot Smith and others occurs in the Mammalia studied for this 

 paper. 



7. Conclusions 



1. The primary arches consist of the paraphysal arch, the 

 postvelar arch, the epiphysal arch and the pars intercalaris 

 (synencephalic arch) and together with the velum are formed 

 in the roof of the forebrain of early sheep embryos. 



2. The paraphysis can be followed in practically all sheep 

 embryos from 20 mm. up to 48 mm. It is characterized by its 

 short, broad and irregular outline and its solid structure, the 

 cavity being in most cases reduced to a minimum. 



3. The diencephalic choroid plexus and lateral telencephalic 

 plexuses are well marked and develop essentially as described 

 in other vertebrates. There is no trace of the median telen- 

 cephalic plexus so noticeable in Amphibia. 



4. The epiphysis forms a short hollow stalk with thick walls 

 and inclined slightly backward over the posterior commissure. 



5. The superior and posterior commissures are formed as in 

 other vertebrates. The posterior commissure is characterized 

 by its precocious development and by the extent that it invades 

 the pars intercalaris of the forebrain in early embryos. 



PART II. HUMAN EMBRYOS 

 1. Primary arches in the roof of the forebrain 



These structures have not as yet been described in human 

 embryos and their present demonstration fulfils the prophecy 

 of the late Professor Minot made in 1901 that they would even- 

 tually be found in all vertebrates, Minot (16). In an embryo 

 of 10 mm. (fig. 11), the primary arches in the roof of the fore- 

 brain are as clearly marked as in any of the lower vertebrates. 

 The postvelar arch is relatively short and thick, while the epi- 

 physal arch is, on the other hand, rather longer than in lower 

 forms. The pars intercalaris is quite extensive and contains 

 in its posterior end a portion of the posterior commissure as 

 was the case in the corresponding stage in sheep embryos. In 



