12 WILLIAM F. ALLEN 



l)iirt to maintain this regular order and in part to add strength 

 to the model, a copper wire was inserted through each section, 

 passing through the point of intersection of the two projection 

 lines. After a certain number of sections had been strung on 

 this wire the}' were securely fastened to each other by pins. 



HEREDITARY AND MECHANICAL CAUSES UNDERLYING THE 



FORMATION OF THE FOURTH VENTRICLE AND THE 



TELA CHORIOIDEA 



To the writer this problem appears to be primarily phylo- 

 genetic rather than ontogenetic; consequently this study begins 

 with the lowest vertebrates, and is approached as a problem of 

 evolution. 



1 . Roof plate of the medulla oblongata 



A. Amphioxus. Unfortunately 1 have not had access to 

 any embryological Amphioxus material so that my inferences 

 will have to be drawn entirely from adult material.- At the 

 outset it can be maintained with a considerable degree of safety 

 that the adult Amphioxus brain contains nothing which can be 

 homologized with the fourth ventricle of higher vertebrates. 

 Figure 67, which pasess through the highest point of the anterior 

 ventricle (F.i) demonstrates clearly that there is nothing here 

 comparable with the fourth ventricle. For the dorsal portion 

 of the cavity is much narrower than the ventral portion, and 

 there is absolutely nothing in the way of a thin and expanded 

 roof plate. In fact, the dorsal portion of the cavity is fairly 

 filled up with processes from ependymal cells. 



A few sections behind the first ventricle there appears a small 

 isolated cavity in the roof plate region of this series, designated 

 as the posterior ventricle (fig. 68, V.2), and a considerable dis- 

 tance behind this cavity, there is a second dorsal cavity, also 

 designated as the posterior ventricle (fig. 69, V.2). This is some- 

 what larger than the previous cavity, and can be readily located 



- In order to eliminate minor details of description from the text, very com- 

 plete and detailed descriptions of the figures have been given at the end of the 

 paper, to which the reader's attention is directed. 



