44 ARTHUR DENDT. 



portion beliind the notochord (fig. 82), so that it is no longer 

 visible from below. The stomodseurn has become widely per- 

 forated by the mouth (fig. 78). Externally one pair of visceral 

 clefts is visible (figs. 74, 75). Transverse sections (fig. 78) 

 show them as a pair of outgrowths of the pharynx which do 

 not, however, as yet perforate the epiblast in the specimen of 

 which sections were cut. Just behind the mouth, and lying 

 above the front part of the pericardium, a median ventral 

 groove in the floor of the alimentary canal is conspicuous in 

 transverse sections (fig. 79, Th.). It is lined by columnar 

 epithelium, and doubtless represents the commencing thyroid 

 gland. 



The two halves of the fore-brain have now completely 

 united, but the line of union can still be recognised in trans- 

 verse sections as a shallow median furrow between the paired 

 eyes (fig. 77, L. U.). 



The auditory pits (figs. 74, 75, 79, Au.) are now well defined 

 and deep, but still widely open to the exterior. 



The optic vesicles of the paired eyes have begun to invagi- 

 nate to form the optic cups (fig. 77, Op. C), and opposite the 

 mouth of each the superficial epiblast is thickening to form the 

 lens (fig. 77, Le.). 



The parietal eye makes its first appearance at this stage as a 

 small rounded diverticulum (primary parietal vesicle) of the 

 roof of the fore-brain just to the left of the middle line 

 and, owing to the cranial flexure, on the apparent ventral 

 surface (vide fig. 7Q, P. V., in which right and left sides 

 are reversed). The number of mesoblastic somites is about 

 twenty-three. 



The head-cavities are conspicuous behind the paired eyes 

 even in the opaque embryo (figs. 74, 75, H. C), and sections 

 show that each has begun to divide into two parts (fig. 77, 

 H. C). 



Wolffian ducts and tubules are both present in the anterior 

 part of the trunk (fig. 80, W. D., TV. T.) ; passing backwards 

 the tubules first disappear, then the ducts also. 



In the front part of the trunk a longitudinal blood-vessel is 



