AVES. 169 



the mesenteron, so that there is in fact a well-developed post- 

 anal section of the gut (fig. 106, p.a.g), which corresponds with 

 that in the Ichthyopsida. For a short period, as mentioned 

 above (p. 163), a neurenteric canal is present connecting the 

 post-anal gut with the medullary tube in the duck, fowl, and 

 other birds. On the ventral wall of the post-anal gut there are 

 at first two prominences. The posterior of these is formed of 

 part of the tail-swelling, and is therefore derived from the 

 apparent anterior part of the primitive streak. The anterior is 

 formed from what was originally the apparent posterior part of 

 the primitive streak. The post-anal gut becomes gradually less 

 and less prominent, and finally atrophies. 



General development of the Embryo. 



It will be convenient to take the Fowl as a type for the 

 general development of the Sauropsida. 



The embryo occupies a fairly constant position with reference 

 to the egg-shell. Its long axis is placed at right angles to that 

 of the egg, and the broad end of the egg is on the left side of the 

 embryo. The general history of the embryo has already been 

 traced up to the formation of the first formed mesoblastic 

 somites (fig. 107). This stage is usually reached at about the 

 close of the first day. After this stage the embryo rapidly 

 grows in length, and becomes, especially in front and to the 

 sides, more and more definitely folded off from the yolk- 

 sack. 



The general appearance of the embryo between the 3Oth and 

 4Oth hours of incubation is shewn in fig. 108 from the upper 

 surface, and in fig. 109 from the lower. The outlines of the 

 embryo are far bolder than during the earlier stages. Fig. 109 

 shews the nature of the folding, by which the embryo is con- 

 stricted off from the yolk-sack. The folds are complicated by 

 the fact that the mesoblast has already become split into two 

 layers a splanchnic layer adjoining the hypoblast and a somatic 

 layer adjoining the epiblast and that the body cavity between 

 these two layers has already become pretty wide in the lateral 

 parts of the body of the embryo and the area pellucida. The 

 fold by which the embryo is constricted off from the yolk- 



