60 



De Vriese's work is even more important than Beddard's. Her paper 

 (21) on the aa, cerebrales anteriores hardly bears on the present 

 problem but one general conclusion is of sufficient interest as to bear 

 quotation. She states that in the vertebrate kingdom the aa. cere- 

 brales anteriores arrange themselves in three distinct groups: 



1) Both arteries run parallel without any communication in fishes, 

 birds, some amphibians and reptiles. 



2) Both arteries run parallel but are united in front of the chiasma 

 opticum by one or more aa. communicantes anteriores in some amphibia 

 and mammals. 



3) Both arteries unite in front of the chiasma to form a single 

 unpaired a. cerebralis anterior communis, which again divides, after 

 a longer or shorter course into two branches, this arrangement being 

 the commonest type and the one found in most reptiles and mammals. 



From a study of the development of these arteries in rabbit embryos 

 De Vriese concludes that: 



1) Both the aa. cerebrales anteriores run parallel. 



2) Both arteries become united by a transverse vascular network. 



3) Both arteries are united together by an a. communicans anterior 

 and there are further three deeper lying aa. cerebrales anteriores — 

 two parallel and one unpaired. 



4) The two aa. cerebrales anteriores fuse to form a single unpaired 

 trunk, which is produced by atrophy of one of the primitive aa. cere- 

 brales anteriores and of the a. mediana. 



Following up this interesting work on the vessels at the anterior 

 end of the circulus arteriosus (Willisi) De Vriese (22) has shown 

 that the primitive brain arteries are branches of the internal carotids 

 and that in all cases in which, in the adult, the brain arteries are 

 derived either from the aa. vertebrales or from a reticular network 

 there has occurred a more or less complicated transformation in con- 

 sequence of which the carotids have become replaced or ampHfied by 

 some other artery standing in relation to them. De Vriese's ideas 

 concerning the morphological changes referred to are illustrated in figure 

 two. She supposes that the internal carotid primitively divided into 

 two well marked terminal branches, cranial and caudal. The aa. cerebri 

 anterior et media are derived from the cranial terminal branch. The 

 a. basilaris is the primitive continuation of the caudal terminal branch 

 of the a. carotis interna; it is originally doubled and is continued on 

 to the spinal cord with branches from the segmental arteries. During 

 the progress of development the a. basilaris becomes unpaired, and 

 later — in most mammals — becomes taken over by the aa. verte- 



