442 Development of the Tuatara (Sphenodon punctatum). 



derm close to the sinus terminalis. The " posterior amniotic canal " 

 thus formed is lined by epiblast, but it lies embedded in the meso- 

 blast of the serous envelope which gradually splits off from the 

 underlying yolk-sac around the embryo. The posterior amniotic 

 canal arises at a very early date, and does not persist very long-. 



(8) The connection between the true amnion and serous envelope 

 (false amnion) in the mid-dorsal line persists in part to a very late 

 stage, but there is free communication between the two halves of the 

 pleuroperitoneal space above the embryo. 



(9) In connection with the vitelline circulation, very numerous 

 absorbing vessels are developed which dip down far into the yolk, 

 and large transparent globules of yolk, each surrounded by a layer of 

 yolk " crystalloids," become arranged around these vessels like onions 

 on strings. The yolk thus gradually assumes a very characteristic 

 radially columnar structure. 



(10) The parietal eye commences its development shortly after 

 the appearance of the optic lobes. It arises by evagination of the 

 roof of the brain in front of the prominence of the mid -brain, and 

 is at first situated slightly to one side of the median line (the left 

 side, so far as yet ascertained). It very soon becomes completely dis- 

 connected from its stalk as a closed, hollow vesicle, the wall of which 

 is composed at first of a single layer of columnar cells. The outer 

 (upper) part of the wall of the vesicle is thickened to form the lens 

 and the inner (lower) part presently divides into two very distinct 

 layers, and acquires a secondary, fibrous connection with the brain 

 immediately in front of the stalk. It is a curious fact that while 

 the parietal eye, after separating from its stalk, at first lies on the 

 left side the stalk itself is median. 



(11) The posterior commissure arises just in front of the place 

 where the stalk of the parietal eye connects with the brain and the 

 stalk passes forwards above it. This fact seems to exclude the 

 possibility of the stalk of the parietal eye representing the pineal 

 gland, for, according to Balfour, the posterior commissure arises 

 behind the pineal gland which is directed backwards. 



(12) The pineal gland in Sphenodon appears to be represented by 

 a mass of convoluted tubules lying in front of the stalk of the 

 parietal eye. 



(13) At a late stage of development (in embryos estimated at 

 from four to eight months) the body and part of the head are . 

 marked with very distinct longitudinal stripes of white on a grey 

 ground. This striping almost entirely disappears before hatching, 

 being last retained on the under surface of the head. This observa- 

 tion is in close agreement with those of Eimer on the markings of 

 mammals, &c. 



(14) In embryos of the same age a patch of cornified epidermis 



