GENERAI. DESCRIPTION. 



The general views of the brain, including the mesal aspect, are 

 reconstructed from camera lucida drawings of transections cut through 

 the entire head. These reconstructions were corrected, as far as possi- 

 ble, by comparison with camera lucida drawings of both frontal and 

 sagittal sections through the head. The details were in all cases studied 

 with higher magnification than was used in drawing. Some of the sec- 

 tions shown were cut a trifle obliquely, and were chosen because slight 

 differences in level often exhibit transitions of form and structure which 

 are instructive. 



No attempt has been made to accurately define the limits of the five 

 segments, because several of the questions of homology involved are 

 stUl unsettled. The general arrangement of the cells is represented but 

 with no attempt to show accurately their size. The membranes, dura 

 arachnoid, pia, blood vessels and capillaries are usually shown at the 

 left, the right of the brain being free. The skull, for lack of space, is 

 not represented. 



The magnification of each figure is given in the explanation. 



PIRATE I. FRONTISPIECE. 



From photomicrographs taken by Simon H. Gage. 



Fig. I. Frontal section of the head of a large red Dieniydylus viri- - 

 descens at the level of the portae. X lo. It shows the brain cavities as 

 continuous from myelocoele to rhinocoeles, the extent of the cinerea, 

 and more indistinctly the plexuses. Owing to the hardness of the lens 

 it could not be cut and the eye was torn in its removal (cf. Fig. 35-40). 



There appears a trace of the intermaxillary gland and latero-caudad 

 of the ear a deeply stained mass the nature of which is not known but 

 which is found in all post embryonic stages examined (cf. Fig. 40). 



The angles in the cephalic cavities show the caudal limits of the 

 rhinocceles. The portse are wide, a horn of cinerea, probably repre- 

 senting Amnion's horn (p. 263) extends latero-caudad from each while 

 directly caudad are the white masses, the dorsally directed columns of 

 the callosum. 



Between the cephalic part of the ear and the brain, the open space is 

 part of the endolymphatic sac and the lateral recesses of the epicoele 

 come into close contact with this sac. 



Fig. 2. A similar section from a larval diemyctylus i cm. long. X 50. 

 Contrast the small relative amount of alba, the membranous mesal 

 walls of the cerebrum, the wide dia- and mesocoeles, the extreme 

 cephalic and lateral extension of the lateral recesses of the epicoele, 

 and the fact that no trace of columns of the callosum can be seen. (cf. 

 Fig. 69, 71). The level of the section is shown at 2 Fig. 67. 



