PINEAL REGION IN MAMMALIA 79 



the postvelar arch inchnes forward and upward above the 

 organ. A swelUng on its right extends into the cavity of the 

 diencephalon. This represents the right half of the diencephalic 

 choroid plexus, D.C.P. The velum, V, is indicated by a sUght 

 fold which is much better marked towards the mid-line and is 

 not shown well in this view. This stage was selected on account 

 of the peculiar appearance of the paraphysis, which was the 

 most irregular specimen seen in any of the sheep embryos, 

 though there were several other cases, notably in embryos of 

 21 mm. frontal series, H. E. C. no. 1686; 25 mm. frontal series, 

 H. E. C. no. 1690; and 40 mm. H. E. C. no. 1691, where the 

 structure was almost as irregular but varied in size. Its mark- 

 edly solid formation differentiates it sharply from the tubular 

 and glandular types so characteristic of Reptilia and Amphibia. 

 In the following two embryos no paraphysis could be found, 

 no. 1111, 24.4 mm. transverse series, no. 1240, 26.6 mm. trans- 

 verse series, so that the paraphysis is not absolutely constant in 

 the sheep embryos. Figure 7 gives the details of a section along 

 the line A-B (fig. 6), taken just above the opening into the brain. 

 The thickness of the main part of the organ is well shown and 

 the cavity has gradually faded out into two narrow prolonga- 

 tions, the most peripheral being its main continuation. One 

 of the solid projections on the right is cut separately and both 

 of these on the left. The general detail of structure can be 

 seen in the drawing and compared with that of the brain wall 

 and the double commencement of the diencephalic plexus also 

 can be made out, D.C.P. 



In an embryo of 34 mm., H. E. C, no. 1692, there is only a 

 rudimentary paraphysis not as large as that shown in figure 4, 

 while in an embryo of 40 mm., H. E. C. no. 1691, the organ is 

 relatively small as compared with that shown in figure 5 and 

 has a central projection with two smaller ones in front and behind 

 which are nearly solid and contain only a slight trace of a cavity. 

 The last three figiu-es are taken from the oldest specimen in the 

 collection, an embryo of 48.4 mm., H. E. C. no. 1696. In figure 

 8, representing a model of the forebrain, there is a picture of 

 the paraphysis in median section, and in figure 9 a \dew of the 



