Blake, Roof of the Foiuth Ventficle. 87 



Up is fused to the oblongata remains and is known as the li- 

 gula ; compare Fig. 31 with Fig. 2. 



In comparing the transections, Figs. 31, 32, 33 and 34, 

 with Fig. 17, the ventricular surface of the Hp appears lying 

 between the ala cinerea and the ligula, and is termed by Retzius 

 the area postrema ; when followed cephalad it is seen to merge 

 into the area acustica. It is separated from the ala cinerea by 

 a slight ridge called by Retzius the funiculus separans, which 

 appears to be at the point of bending of the lip. According 

 to Retzius the area postrema differs considerably in individuals, 

 which is also characteristic of the secondary lip. 



Just before the region of the lateral recess is reached, 

 frontal sections show a commencing reduplication of the lip. 

 Figs. 3 and 7. The lip here is folded laterad, mesad, then 

 again laterad and finally mesad to end in the tela. In other 

 sections these folds are seen to be fused together so that the 

 ventricular surface comes to be narrowed to some extent. At 

 the same time it is evident that the endymal layer is carried lat- 

 erad and enclosed in the substantial nervous matter of the ob- 

 longata. 



The foldings of the secondary lip are not so pronounced 

 in the lower animals but are recognizable. In Fig. 9 a frontal 

 section of the brain of a cat embryo ten millimetres N. B. 

 measurement, the folding of the lip may be seen at the margin 

 of the ventricle as a slight thickening. An analysis of this 

 with a higher power shows a condition represented in Fig. 10. 



The cells thus transferred to the ectal surface of the oblon - 

 gata seem to give rise to a group of cells which appear on the 

 sides of the oblongata in many embryos as far cephalad as the 

 trigeminus and which may be connected with the ganglia of 

 some of the nerves. An apparent connection was observed in 

 relation to the auditory and it seemed from their position that 

 they might have a possible connection with the so-called acces- 

 sory nucleus of that nerve. I have been unable, however, to 

 obtain sufficient evidence to make a positive assertion in regard 

 to them. 



At the region of the lateral recess the secondary lip un- 



