140 



GENERAL DEVELOPMENT OF THE EMBRYO. 



Sp.C 



is in fact a well-developed post-anal section of the gnt (fig. 106, p.a.g), 



which corresponds 

 with that in the 

 Ichthyopsida. For 

 a short period, as 

 mentioned above 

 (p. 135), a nenren- 

 teric canal is pre- 

 sent 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. 



^y- 



Fig. 106. Diagrammatic longitudinal section through 

 thk posterior end of an embryo bird at the time of the 

 formation of the allantois. 



ep. epiblast ; SjJ.c. spinal canal; ch. notochord; ii.e. 

 neureiiteiic canal ; hij. hypoblast; p. a. f/. post-anal gut ; 2^r. 

 remains of primitive streak folded in on the ventral side ; 

 al. allautois ; me. mesoblast ; an. point where anus will be 

 formed ; p.c. perivisceral cavity : am. amnion ; so. somato- 

 pleure ; sp. splanchnopleure. 



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 (tig. 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 80th and 40th 

 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 constricted 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 hypo- 

 blast and a somatic layer adjoining the epiblast — and that the body- 



