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On the Morphology of the Pineal Kegion 



physal arch forms a distinct evagination, P, with thin walls and its top 

 is irregular as if the walls had formed the anlages of new outgrowths. 

 I am thus led to believe that the arch has not at this stage yet com- 

 pleted its differentiation, but on the contrary is about to form the true 

 paraphysis, or paraphysal gland as a local development of the arch 

 proper. My efforts to obtain more advanced stages having failed I must 

 leave the decision to future observations. My belief, or better said — 

 supposition, is confirmed by the fact that in amphibians and birds the 



Fig. 10. Embryo of 86 mm. Sagittal series, 436, section 293. x 30 diams. 



})araphysal gland is an appendix of the arch — see below. The velum, 

 V, has now distinctively the character of a choroid plexus being very 

 irregular in the form of its surface, rich in blood vessels, covered by a 

 thin ependyma and projecting far into the cavity of the brain. Later- 

 ally the projections from its surface are much more developed, and as 

 the organ has grown forward alongside the median paraphysal arch, it 

 has produced what we can now easily identify as the plexus of the 

 lateral ventricles. These plexuses are therefore to be interpreted mor- 

 phologically as secondary modifications or appendages of the primary 



