PINEAL REGION IN MAMMALIA 95 



arch is long and flat, the epiphysal arch just developed and the 

 pars'intercalaris of relatively large size with the posterior com- 

 missure partly in it and partly overlapping into the midbrain. 

 The next stage to be described is an embryo of 26 mm. shown 

 in figure 24. The paraphysal arch, P.A., is reduced to a deep 

 narrow fold which passes over into the velum, V, which has 

 been wholly involved in the plexus formation. The roof plate 

 above the velum swells forward over the telencephalon but this 

 seems to be chiefly choroidal in character and is similar to what 

 is seen in rat embryos (figs. 25 and 26) and especially in cat 

 embryos (figs. 34 and 35). As is the case in the cat, the plexus 

 has involved all of the diencephalon up to the deep suprapineal 

 recess. A peculiar feature of this stage is the absence of an epi- 

 physis, only the low arch just behind the pineal recess repre- 

 senting it. This however develops later. See Jordan (13) 

 who gives a good account of the histology of the organ. The 

 commissure shown here is the posterior commissure which fills 

 up all of the pars intercalaris. 



This was the oldest stage in the Collection with the ex(5eption 

 of a set of sections through the head of an opossum after birth 

 which were in too poor condition to be of any value. The orig- 

 inal description of the paraphysis was given by Selenka in the 

 opossum, but he contented himself with making the statement 

 that it existed and gave no pictures of it as far as I can discover. 

 It is hoped that some investigations will be published on older 

 embryos and on the adult opossum in order to confirm the pres- 

 ence of the structure in this species. 



2. Rodents 



A. Rat. The Harvard Collection contains an excellent series 

 of these embryos up to 25 mm. in which the earlier development 

 of the pineal region can be advantageously studied. Figure 25 

 shows the primary arches in an embryo of 9.6 mm. with the 

 posterior commissure partly in the pars intercalaris of the fore- 

 brain, as is always the case in Mammalia. An embryo of 

 14.4 mm. is shown in figure 26. In the paraphysal arch there 



