DEVELOPMENT OF THE CHOROID PLEXUS 101 



of His probably represents all the membranous parts anterior 

 to the velum trans versum. 



g. Lamina supraneuroporica. Between what is obviously 

 lamina terminalis and what is just as obviously tela chorioidea 

 telencephali medii in the 28 mm. embryo and the 32 mm. em- 

 brj^o, lies the peculiar thickening of the roof plate before noticed 

 Cfig. 6, t.r-p.) which seems by a process of elimination to be lamina 

 supraneuroporica. Whether this interpretation is correct, I 

 cannot say, because I have not had material with which to fol- 

 low this region in its later development. There is in the 32 

 mm. embryo, a short portion of the roof plate between the deep 

 pit (fig. 25, d.p.) and the shallow recess on the ventricular sur- 

 face (fig. 25, r.n.?) which may be lamina supraneuroporica. 

 In the 19 mm. embryo, the lower limb of the greater arch (fig. 

 18, l.s.f) seems its most likely location. 



h. Plexus chorioideiis ventriculi lateralis. The problem of 

 the formation of the lateral choroid plexuses is one of consider- 

 able difficulty, because of the complex morphological relations 

 involved, but if a few facts of development be remembered, 

 the problem l)ecomes relatively simple. 



If the brain of a half-grown frog tadpole be examined, it will 

 be found that "the membranous roof of the forebrain ventricle 

 is attached to the massive wall of the hemisphere by the taenia 

 fornicis which is directly continuous caudad with the taenia 

 thalami" (Horrick, '10). The taenia thalami is the attachment 

 of the roof i)late posterior to the velum transversum to the 

 lateral wall of the thalamus. It might be added also that the 

 taenia fornicis becomes continuous with the taenia thalami 

 at the lateral end of the velum transversum. In the middle 

 of the membranous roof in front of the velum transversum arises 

 the paraphysis. There is no plexus lateralis in the larval or 

 adult frog, but in urodele Amphibia it is between the paraphysis 

 and the taenia fornicis that the lateral choroid plexus makes its 

 appearance, pushing into the ventricle. Warren ('05) in de- 

 scribing the development of the lateral plexuses of Necturus 

 maculatus says, "The telencephalic plexus develops from the 



