486 ERNEST W. L, HOLT, 



Immediately behind these the S"^* or oculomotor nerves pass out from 

 the centre of the crura cerebri. The relations of the infundibulum 

 to the pituitary body (fig. 9 pt.) remain unchanged. The former ex- 

 tends back as a thin-walled sac {in.) some way above the notochord ; 

 its walls are here somewhat plicated, a condition which appears to 

 become much more marked in Anarrhichas (cf. 6, p. 915, pi. XXIII, 

 fig. 4). 



III. ^ inch stage. 



Plate XXX, fig. 17. Plate XXXI, fig. 20. Woodcut-fig. 3. 



Since the last stage the cerebrum has undergone a further up- 

 ward rotation, and the olfactory lobes are now seen as bulbous masses 

 projecting from its front end. From the anterior ventral edge of 

 each lobe the olfactory nerve passes outwards to the nasal mucous 

 membrane. Each olfactory lobe consists, histologically, of fibrous 

 matter, with a few scattered deeply staining cells, and a narrow peri- 

 pheral margin (a condition very similar to that met with in the next 

 stage and shown in figure 15). It is constricted at its point of origin 

 from the cerebrum, as in later stages (cf. fig. 18 I. öl.\ but its height 

 is as yet insignificiant. 



Changes have occurred in the pineal region. The anterior ends 

 of the lateral optic ventricles (fig. 20 v. o.) are seen divided by a pale 

 median septum (s.), resting ventrally on the fibrous tract (of which 

 the lateral extremities are seen at t.f.) bridging over the third ven- 

 tricle. The septum breaks down before the posterior end of the tract 

 in reached. It is treated at greater length in the next stage. 



Fibres from the roots of the optic nerves can be easily traced 

 up into the fore part of the optic lobes. The thin cellular roof of 

 the optic ventricle (fig. 20 t. c.) shows a marked downward folding 

 in the middle line. The ventral commissure of the infundibulum is 

 unchanged. 



White matter enters more largely into the composition of the in- 

 fundibulum than formerly. After the splitting off of the latter, a broad 

 belt of white matter, taking the place of the posterior region of the 

 dividing commissure, forms the ventral part of the cerebral mass 

 (crura), and becomes connected on either side with the white matter 

 of the tectum lobi optici. 



The lumen of the infundibulum becomes trifid, as in Anarrhichas 

 (cf. 6, p. 915) having an inverted T- shape in transverse section. Its 



