46 ARTHUli DliNDY. 



it has already begun to turn downwards and acquire its cha- 

 racteristic spiral twist. 



The Wolffian ridges (figs. 83, 85, W. R.) have made their 

 appearance, and are quite conspicuous at the hinder end of the 

 body. 



The cerebral hemispheres (figs. 83, 90, 91, C. H.) have 

 begun to grow out from the fore-brain, causing a marked 

 elongation of the head in front of the eyes. 



The primary parietal vesicle (fig. 83, P. K) is prominent, 

 still in the form of a simple hollow outgrowth of the loof of 

 the thalamencephalon, causing a projection of the external 

 epiblast between the paired eyes. The cavity of this vesicle 

 still communicates freely with that of the brain; its wall is 

 composed of a layer of columnar cells, like those forming the 

 brain roof, and is not yet thickened to form a lens. 



The auditory vesicles (figs. 83, 84, 91, Ati.) are almost if not 

 quite closed. 



In the paired eyes, pigment is just beginning to be visible 

 in living specimens, but it is not yet noticeable externally in 

 the preserved embryo. 



The nasal pits (fig. 83, Na.) are just beginning to appear. 



The first three visceral clefts are visible externally (figs. 83, 

 84), and four are recognisable in sections (fig. 91). 



Perhaps the most characteristic feature of this stage is the 

 first appearance of the allantois, which is visible externally as 

 a small rounded or finger-shaped outgrowth (figs. 83 — 86, All.) 

 on the ventral aspect of the body beneath the root of the tail 

 and between the two Wolffian ridges, and projecting into the 

 pleuro-pevitoneal space above the yolk-sac. It originates as a 

 very thick-walled outgrowth from the posterior enclosed part 

 of the alimentary canal, its walls being continuous behind with 

 the Wolffian ridges. The transverse sections figured (figs. 87 

 — 89), of which that represented in fig. 89 is the most 

 anterior, show this mode of origin sufficiently without further 

 explanation. 



The closure of the alimentary canal posteriorly has extended 

 forwards for some little distance, and the enclosed portion 



