56 ARTHUR DENDY. 



The short stalk of the allantois passes out on the right side 

 of the embryo, and the allantoic vessels at once divide into two 

 sets. One set passes upwards just in front of the right arm in 

 that part of the allantois which adheres closely to the true 

 amnion, while the other passes downwards in that part which 

 adheres to the yolk-sac. The vessels of the upper set are 

 reflected back above the embryo into the outer limb of the 

 allantois, under the serous envelope. 



As already stated, the allantois extends almost completely 

 round inside the serous envelope, but it is of course interrupted 

 by the attachment of the latter, on the one hand, to the true 

 amnion above, and on the other to the yolk-sac below. 



The yolk-sac is now attached to the ventral surface of the 

 embryo by a slender stalk about 5 mm. long. The vitelline 

 circulation embraces nearly the whole of the yolk, but there is 

 a rounded area, about 6 mm. in diameter, on the side opposite 

 to the embryo, over which the vitelline circulation has not yet 

 extended. Although this non-vascular area is well defined, 

 there is no longer a distinct sinus terminalis. It is in this 

 region, of course, that the serous envelope is still united with 

 the yolk-sac. 



The radially coluiiinar structure of the yolk, due lo the 

 aggregation of the yolk particles around the absorbent vessels, 

 as described in the Introduction, is now becoming well marked. 



For external characters the embryos of July may be taken as 

 typical. They present the following strongly marked advances 

 on the preceding stage : 



(1) The head has acquired a markedly Chelonian form, 

 which may be better realised by reference to figs. 104 and 105 

 than from any description which I can give. 



(2) A very conspicuous, sharp-pointed "shell-breaker" (fig. 

 104<,S. B.) has been developed on the snout by thickening and 

 cornification of the epidermic cells. 



(3) The external naves are represented by a pair of small 

 round white spots (fig. 104, Na.), liaving been completely 

 blocked up by a dense growth of cells which fills the outer 

 part of each nostril, and which is already present at the com- 



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