THE DIENCEPHALIC FLOOR 255 
In the mammillary region the processes initiated in the stage 
of eight days have now proceeded to such a degree that the 
lateral mammillary recesses have become obliterated and the 
mamunillary bodies are now solid, containing no recess accessory 
to the third ventricle. There is’ still a slight remnant of the 
median mammillary recess (26), but this also has been consider- 
ably reduced in size. 
17 31 
Fig. 22 Mesial view of forebrain reconstruction of 3 mm. Mustelus laevis. 
xX 150. The unshaded area shows the cut surfaces of the reconstruction. 17, 
infundibular region; 25, mammillary region; 28, neuropore; 29, optic vesicle 
or evagination; 31, optico-infundibular groove; 45, tuberculum postero-superius; 
46, tubercle of the floor of*Schulte. ; 
Development of the diencephalic floor in Mustelus laevis 
Mustelus embryo of 3 mm.; Specimen No. 722 (fig. 22). The 
embryo of this stage corresponds closely to the cat embryo of 
eight somites and is also similar to the chick embryo of that 
size. The neuropore (28) is still widely open, while the optic 
vesicle (29) forms the only structure at the cephalic extremity 
of the neural tube. Ectally this vesicle is represented by a 
marked lateral protuberance of the neural wall whose axis is 
oblique and whose apex is directed caudad. The surface relief 
of the brain at this stage is shown in figure 23. Dorsal to the 
