PINEAL REGION IN MAMMALIA 81 



4. Postvelar arch 



At first the postvelar arch forms a short curve in the roof of 

 the first diencephahc segment and later with the appearance of 

 the epiphysal arch becomes more elongated. The diencephahc 

 choroid plexus begins to form at the velar end of the arch and 

 appears at first in embryos of about 25 to 26 mm. (fig. 4, D.C.P.). 

 The plexus develops at first on either side of the median line 

 forming two marked tufts of tissue bulging into the diencephalon 

 and beginning just behind the diencephalic leaf of the velum. 

 After a short distance the separate tufts become more or less 

 condensed into a single median tuft which invaginates the di- 

 encephalic roof plate in the mid-line {D.C.P., figs. 6 and 7). 

 In an embryo of 32 mm. the plexus has involved a large part of 

 the postvelar arch and in the oldest specimens has extended 

 dorsally nearly to the epiphysis. The size and character of this 

 formation are well shown in figure 10. The lateral choroid 

 plexus is first seen in embryos of about 20 mm. arising on either 

 side of the telencephalic roof plate and of the opening of the 

 paraphysis. Bailey in two excellent papers, the one on the 

 ''Morphogenesis of the Choroid Plexuses," the other on the 

 ''Morphology of the Roof Plate of the Forebrain and Lateral 

 Choroid Plexuses in the Human Embryo" has considered so 

 carefully the origin of the plexuses, especially that of the lat- 

 eral plexus, that it is unnecessary to add anything further here. 



5. The epiphysis 



The epiphysal arch appears first in an embryo of about 14 mm. 

 (fig. 2) and is relatively short but well marked. It continues in 

 approximately this condition up to about 24 or 26 mm., when 

 its walls thicken and it becomes more elevated above the sur- 

 face of the brain. It now may truly be called the epiphysis and 

 both commissures are placed close against its anterior and pos- 

 terior walls (fig. 4). In figure 5 it is more clearly defined and 

 begins to incline slightly backward a position which is more 

 strikingly shown in the oldest embryos (fig. 8). In the speci- 

 mens studied there was no sign of the differentiation of a pineal 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 1 



