Blake, Roof of the Fourth Ventiicle. 97 



tions of this region. I should expect in these forms to prob- 

 ably find a caudal protrusion of the ventricle. 



In Aves I have found in embryo chicks of the tenth day 

 and later a very marked caudal protrusion which is received in 

 a special compartment of the skull. 



In comparing the conditions in the roof of the body of 

 the ventricle in man, with those found in the lower animals, the 

 similarity is seen to be very striking, except for the difference 

 already mentioned in the development and extension of the 

 secondary rhomboidal lip. 



The relation of the vermis is practically the same as is 

 found in advanced embryos of other animals, namely, it does 

 not extend over and cover in the caudal extremity of the ven- 

 tricle. We have then the conditions which in other animals 

 have been shown to be accompanied with a caudal extension of 

 the roof forming a closed pouch. In the foetus at the begin- 

 ning of the fifth month frontal sections of this region show the 

 following appearances : if the sections are followed back from 

 where they pass through the cavity of the ventricle in the first 

 sections will be seen the floor of the ventricle, or either side 

 the secondary lip folded laterally and extending from its margin 

 a wall of epithelium with its pial substratum, which passes dor- 

 sad toward the cerebellum and meets with its fellow in the roof 

 where the choroid plexus is seen. Thus we have a condition 

 of complete coelian circumscription. Fig. 28. Passing caudad 

 to the apex of the ventricle we find that a complete endymal 

 tube is now cut in whose roof the choroid plexus is still seen, 

 and whose floor is separated from the medulla by a reflexion of 

 the pia. Fig. 27. We have here the same picture as seen in 

 sections of the same region in advanced embryos of the dog, 

 pig, cat, and sheep. Figs. 22 and 23. 



The walls of the tube, however, are much thinner and its 

 caliber greater. In following the sections further caudad, the 

 walls soon become lost, but the roof still continues fused to the 

 extension of the pia under the cerebellum. Fig. 26. 



Hess describes the brain of a child at term in which he 

 states that the ventricle was closed by a pouch-like fold of the 



