84 K. K. Oliver: 



Stage V. (PI. XXIX.) 



The fifth stage corresponds approximately to a period of 

 twelve days' incubation. 



On viewing the Sipecimen dorsally (5b) marked changes will 

 be seen to have taken place since the last stage. The anterior 

 portions of the cerebral hemispheres are now more closely 

 opposed to each other than are the posterior parts. The optic 

 lobes are becoming so rotated that their longitudinal vertical 

 axes are almost at right angles to the axis of the cerebral 

 hemispheres. The cerebral hemispheres are beginning to rise 

 up dorsally, so that they overshadow the diencephalon and the 

 epiphysis somewhat, and the pressure exerted on the optic lobes 

 by rhem is beginning to force the optic lobes apart. Dorsally, 

 the optic lobes are still prominent, but not so much so as at the 

 preceding stages. So far it has been rather that the surround- 

 ing parts have grown up to the level of the optic lobes, than 

 that they themselves have grown down, but from this stage on, 

 the optic lobes descend steadily, presumably owing to the pres- 

 sure brought to bear on them anteriorly by the cerebral hemi- 

 spheres first, and then, later, posteriorly by the dorsal develop- 

 ment of the cerebellum. The cerebellum at this stage has 

 become distinctly indicated, and is beginning to assume its 

 tj^pical convoluted appearance. The dorso-lateral growth of the 

 metencephalon has increased in such a manner as to begin to 

 curl over on itself towards the myelencephalon, while on the 

 dorsal surface of this area there are some gix faintly marked 

 convolutions appearing. The myelencephalon at this stage is 

 also l)eginning to assume the characteristic appearance of the 

 medulla oblongata, and both cerebellum and medulla are clearly 

 seen from the dorsal aspect, the cranial flexure having changed 

 so that the axes of fore, mid and hind brain now lie along 

 approxiately the same straight line. 



The lateral view (5a) shows a distinct change in shape. The 

 diencephalon is now completely hidden by the pushing out of the 

 optic lobes to the sides. The change in the direction of the 

 axes of the lobes is much more clearly seen at this stage than at 

 the preceding one. The epiphysis is hidden by the growth of 



