82 



Ou the Morphology of the Pineal Kegion 



in the roof of the diencephalon are just beginning. Fig. 1 represents 

 a sagittal section, the plane of which is not true since it passes through 

 the epiphysal anlage about in the median plane, but in the region of the 

 hind brain passes considerably to one side so that it strikes the jugular 

 vein, Ve, the first aortic arch, Ao, and the three " head cavities," the 

 praemandibular, 1, the mandibular, 2, and the hyoid, 3. Miss Piatt's 

 " anterior cavity," 91.2, 201, is also shown, and is connected in sec- 

 tions 88 and 89 of this series with the praemandibular cavity. In our 

 transverse series, No. 206, section 120, the two cavities are just sepa- 

 rated. It seems, therefore, probable that the " anterior cavity " arises 



Fig. 1. Embryo of 11.5 mm. Sagittal series, No. 208, section 93. 

 (For explanation of the lettering see at end of the article.) 



X 30 diams. 



from the praemandibular as van Wighe found to be the case in Galeus. 

 It may be noted that the tip of the notochord joins the wall of the 

 praemandibular cavities, which at this stage are still united across the 

 median line.* The fore-brain is already subdivided into the wider 

 anterior prosencephalon and the narrower posterior diencephalon. The 

 roof of the latter has two well-marked arches, of which the posterior, 

 Ep., is the anlage of the epiphysis. In connection with these arches 

 appear on the inside of the brain wall in the section three points; of 



3 The condition of the head-cavities is described as it offers a convenient means of 

 fixing the stage at which the dififerentiation of the pineal region begins. 



