230 CHESTER H. HEUSER 



to this one show the fold to be continuous from one recess to 

 the other. The marked villous character of the chorioid plexus 

 is illustrated well in this section (fig. 23). It shows also the 

 rhomboid fold projecting into the cavity of the hind-brain below 

 the chorioidal lamina. The rhomboid fold appears in the model 

 (fig. 15) as a sharp rim, extending along the upper part of the 

 body of the ventricle behind the lateral recess. It extends to 

 the caudal protrusion of the fourth ventricle, which has expanded 

 to a very great size; its volume is more than one-half that of the 

 remainder of the ventricle. This protrusion, from above and 

 from behind, appears almost spherical. The posterior boundary 

 slopes downward making almost a right angle with the narrow 

 cavity of the caudal part of the medulla. When the body of 

 the fourth ventricle is viewed from below, the ventral median 

 sulcus and the two sulci limitantes are seen converging towards 

 the cervical flexure. The ventral median groove is nearly straight 

 and produces with the slim cavity of the cord an angle of about 

 125°. 



In the 260-mm. embryo (figs. 25 and 26) the cavity of the 

 isthmus is very broad and flat. On the ventral surface a deep 

 ridge — the median ventral sulcus — is continuous with the similar 

 ridge of the mid-brain ventricle. On account of the extensive 

 growth of the cerebellum the dorsal surface of the body of the 

 ventricle has become deeply concave. Only a very slight caudal 

 protrusion is now present. Bradley ('05) described the first ap- 

 pearance of openings in the lateral recesses — foramina of Luschka 

 — ^in embryos of about 100 mm. Because of these openings and 

 on account of the growth of the chorioid plexus the cavities of 

 the lateral recesses, as seen in the model, have been reduced to 

 slim irregular bodies. 



MEASUREMENTS OF THE BRAIN 



In order to show the change in the volume of the cavities 

 during development, measurements of wax models were made 

 by the immersion method. The results of these measurements, 

 reduced to cubic milhmeters, are given in table 1. 



