'MORPHOGENESIS OF THE CHOROID PLEXUSES 511 



paired masses growing backward from the origin of the lateral 

 plexus into the diencephalon" of which Warren writes do not 

 seem to me to be at all homologous with the median telencephalic 

 plexus. They arise too far posterior on the plexus; are at no time 

 connected with the roof plate; and are merely prolongations of 

 the lateral plexus. The diencephalic plexus is not so well devel- 

 oped as in Urodeles but is present in all forms, as is also the plexus 

 of the fourth ventricle. Humphrey ('94) shows in Chelj'dra 

 serpentina the lateral telencephalic plexus arising from the base 

 of the median telencephalic plexus and passing into the lateral 

 ventricle. According to Warren ('11), in Chrysemys marginata, 

 "The plexus chorioideus laterahs springs from the paraphysal 

 arch immediately in front and lateral to the mouth of the para- 

 physis, figure 25, and invaginates the dorso-mesial wall of the 

 hemisphere." 



In considering the Mammalia, considerable space will be given 

 to interpreting accurately the velum and paraphysis. This is 

 rendered necessary if one is profitably to homologize the lateral 

 telencephalic plexus of Mammalia with the same structm-e in 

 the lower vertebrates. Since, as we shall see later, the jiarai^h}'- 

 sis never appears in Mammalia excei)t as an arch of the roof 

 plate of the telencephalon in early stages of development, it will 

 be referred to as the paraphysal arch. ^Nlinot ('01) used the term 

 'to apply to the entire roof plate of the telencephalon between 

 the velum transversum and the lamina terminalis, a sense in 

 which it is no longer used. 



The only observations of value dealing with the Monotrenies 

 are those of Th. Ziehen ('05) on Echidna hystrix and G. Elliot 

 Smith ('97) on Ornithorhynchus, and these leave much to be 

 desired. Smith describes the origin of the lateral telencephalic 

 plexuses and a structure which he says "constitutes the para- 

 physis of Selenka." I do not know what ex'idence he had for 

 stating that the structure he describes is the paraphysis of Selenka 

 for so far as I ha^'e been able to determine, Selenka has nowhere 

 a description of the paraphysis but merely the bald statement 

 that it is present in Marsupials. Smith's statement probably 

 implied no more than that he interpreted this structure as the 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 26, NO. 5 



