80 JOHN WAREEN 



paraphysis is given practically similar to that seen in figure 6. 

 One is looking from the front and a bit to the right of the mid- 

 line at the outer surface of the lamina terminalis, L.T., and of the 

 paraphysis, P. Behind the paraphysis the diencephalic choroid 

 plexus, D.C.P., bulges backward into the diencephalon. The 

 space between the paraphysis and the wall of the plexus is of 

 course filled wiih vessels and comiective tissue which could not 

 be modeled. The base of the paraphysis is nearly as broad as 

 that shown in figure 6 and is like^^'ise prolonged into a narrow 

 tube which is hollow up to the tip, but no lateral projections 

 are seen. A distinct fold, V, marks the position of the velum. 

 Figure 10 is a section through the paraphysis and the dien- 

 cephalic plexus at the line A-B (fig. 9). The section is through 

 the broadest part of the organ, the wall of which is thick and 

 well defined and stands out in contrast to the cells covering the 

 folds of the plexus. The cells in the paraphysis form a double 

 row while those covering the folds of the plexus are for the most 

 part arranged in a single row. This specimen is hollow and dif- 

 fers in this respect from the more solid types seen in the em- 

 bryos of 29 mm. and 40 mm. and a good view of the character 

 of the tufts of the diencephaUc plexus is given in the picture. 

 The paraphysis is thus shown to be practically a constant struc- 

 ture in embryos of 21 mm. to 48.4 mm. and differs markedly 

 from the elongated tubular organ of Reptilia and the compli- 

 cated glandular structures of Amphibia, its chief characteristics 

 being its short, broad and irregular form and its solid character. 



3. Velum transversuvi 



The velum appears in some of the earliest embryos and can 

 always be clearly followed up to the oldest stages studied for 

 this article. At first it forms a comparatively slight fold in 

 the roof of the forebrain which, with the elongation of the post- 

 velar arch, becomes more accentuated especially towards the 

 median line, but as the choroid plexuses develop it tends to be 

 more obscured. The observations made here agree mth those 

 previously made in other Mammalia, Johnston (12), and with 

 Neumayer's pictures of early sheep embryos (17). See also 

 Ziehen (25). 



