MORPHOGENESIS OF THE CHOROID PLEXUSES 519 



considers the lateral plexus in the guinea pig to be derived 

 from the velum transversum. "E dal velum che si origina il 

 plesso del III ventricolo e i plessi coroidei emisferici destro e 

 sinistro." It is interesting also to note that although Tilney ('15) 

 correctly labels the paraphysal arch in models of the brains of 

 young cat embryos, in later stages he invariably transfers the 

 label to the anterior pouch of the diencephalic roof. Johnston 

 ('09) has correctly interpreted the paraphysis in pig embryos, 

 but his statement that the lateral telencephalic plexus appears 

 as a folding of the anterior wall of the velum transversum is 

 misleading for ho immediately follows with the statement that 

 it is separated from the velum by tlic i^araphysal arch. 



In the angle between the vesicle and the dicncepluilon appears the 

 chorioid plexus pushing into the lateral ventricle. It appears as a fold- 

 ing of the anterior wall or limb^jf the velutn transversum and its lateral 

 prolongation. In this way ai)pe:ir.s the chorioid fissure whose further 

 history need not be traced. Near the median line the plexus apjiears 

 as a fold projecting into tiic interventricular foramen and separated 

 from the velum transversum by the paraphysal arch. 



MATKI{IAL AND .MHTHODS 



The material on which this study is based consists of a series 

 of twelve embryos of Chrysemys marginata from mj'- own col- 

 lection, ranging in size from 5.1 mm. greatest length to one having 

 a carapace 10.0 mm. long, and an 8.8 mm. embryo from the 

 Harvard Embryological Collection, very kindly loaned to me 

 by the Harvard Laboratory. All of my material was fixed in 

 Zenker; stained in bulk with borax carmine; embedded by the 

 celloidin-paraffin method; cut at 10m in transverse series; and 

 counterstained on the slide with orange G. The excellent pres- 

 ervation of the form relations of the delicate membranous por- 

 tions of the brain may be attributed to the method of embedding. 

 The embrj'os were all passed from 95 per cent alcohol to ether- 

 alcohol; then through 0.5, 1 and 2 per cent celloidin; hardened 

 in chloroform alcohol, and cleared in benzol, before embedding 

 in paraffin. The Harvard embryo is cut in 10 m sagittal sections 

 and stained with borax carmine and eosin. 



