Franklin P. Mall 7 



The circle of Willis is fully formed in this specimen and extends from 

 the bifurcation of the basilar artery to the anterior communicating. At 

 the point the carotid enters the circle there is a short ophthalmic which 

 is also present in the embryo of the fourth week (No. 2), and is shown 

 in No. 74, Pig. 5. Throughout the region of tlie brain branches rise at 

 quite regular intervals from the anterior communicating to the vertebral 

 arteries. So regular are these branches that they might be spoken of 

 as the segmental arteries to the brain. These are then gradually shifted, 

 some becoming enlarged and others disappear. 



The anterior and middle cerebral arteries (Pig. 4) arise as a common 

 stem and form a main branch encircling the optic stalk from which 

 small branches pass on the lateral side of the cerebral vesicle while the 

 main stem continues to the front of the brain and communicates with 

 its fellow on the opposite side immediately behind the olfactory pit. It 

 is easy to imagine the anterior cerebral pushed into place when the 

 cerebral vesicle protrudes over it in every direction. In embryo No. 74 

 (Pig. 5) the middle cerebral is much better marked, while the anterior 

 cerebral cannot be followed to its end. In embryo No. 145 (Pigs. 6-8) 

 the adult form of these two vessels is well given. Numerous radiating 

 branches mark the middle cerebral over the embryonic island (Fig. 6) 

 and the anterior cerebral extends to the mesial side of the hemisphere 

 as it does in the adult (Fig. 7). The anterior cerebral artery is pic- 

 tured by His in his last great monograph on the brain ' in an embryo ' 

 of about the same age as my embryo No. 145. This illustration is from 

 a sagittal series like the one from which my Pigs. 6-8 were reconstructed. 

 In His' paper this vessel is called " die vorder Bogenvene die das Blut 

 aus dem vordern Abschnitte der Hemispharenwand sammelt.''* When 

 the direction of this vessel is considered, and especially when it is re- 

 constructed, it is easily shown that His was in error in calling it a vein. 

 The anterior choroidal artery is next in order, for it also arises from 

 the carotid artery and its destination, the choroid plexus, is well marked 

 in young embryos. In Fig. 4, the artery which takes this position is 

 intimately associated with the middle cerebral and lies between the 

 cerebral hemisphere and the optic thalamus. It may be that what I have 

 termed the anterior choroidal is in reality the middle cerebral, and that 

 the artery more dorsalwards is in reality the choroidal, for it is well 

 known that arteries often shift a great deal in young embryos. Not 



= His: Die Entwicklung des menschl. Gehirns. Leipzig, 1904. 

 ' Page 79, Fig. 51. 

 * P. 125 



