258 FREDERICK TILNEY 
Mustelus embryo of 11 mm.; Specimen No. 729 (fig. 25). The 
midbrain flexures are present in this embryo and have caused 
deflections of the neural axis at the anterior and at the posterior 
isthmian sulci. The ectoptic zone now gives more definite 
evidence of its ultimate derivatives, the dorsal segment showing 
Fig. 25 Mesial view of forebrain reconstruction of 11 mm. Mustelus embryo. 
x 100. The unshaded area shows the cut surfaces of the reconstruction. 4, 
chiasm; 7, epiphysis; 18, infundibular evagination; 24, mid-brain; 25, mammil- 
lary region; 29, optic evagination; 36, post-infundibular evagination; 45, tubercu- 
lum postero-superius; 46, tubercle of the floor of Schulte. 
the characteristic thalamencephalic formation, while the cephalic 
segment presents allof the primitive elements of the telencephalon. 
The ventral segment of the ectoptic zone is considerably modified. 
The infundibular region (17) has not only increased in size, but 
at its apex or caudal portion two evaginations have made their 
appearance. Of these the infundibular evagination (18) is 
more extensive but less protuberant. The pituitary anlage is 
