6i6 



BIRDS. 



The precise origin of the constituents of this middle layer is uncertain, 

 but it is important to notice that all three layers of the embryo are con- 

 nected at the sides of the primitive streak, as at the margin of the blas- 

 topore in the frog. 



In the region in front of the primitive streak, a row of hypoblast 

 cells becomes differentiated to form the notochord. At its sides the 

 sheets of mesoblastic cells split into an inner or splanchnic layer, and an 

 outer or somatic layer. A little later the mesoblast divides into the 

 segmentally arranged mesoblastic somites, lying at the sides of the noto- 

 chord, and the unsegmented lateral plate, whose outer and inner walls 

 form the corresponding boundaries of the ccelome. 



At the time when the notochord has appeared internally, the external 

 epiblast becomes differentiated to form the medullary groove, which gives 

 rise in the usual way to the medullary canal. The folds at first diverge 

 posteriorly on either side of the primitive streak, but as the union travels 

 backwards, this is included in the medullary canal, and so disappears. 



During the course of the second day, the embryo seems to sink 

 further into the yolk, while both anteriorly and posteriorly double folds 

 known respectively as the head 

 and the tail folds, rise up. In 

 the course of their development 

 the embryo becomes completely 

 " folded off" from the yolk. At 

 a slightly later stage, side folds 

 also appear ; all the folds now 

 consist of a double layer of soma- 

 topleure covered externally by 

 epiblast. The folds meet above 

 the back of the embryo and 

 coalesce. The inner layer forms 

 the true amnion, the outer the 

 false amnion or subzonal mem- 

 brane. Into the space between 

 the amniotic folds, an outgrowth 

 from the posterior region of the 

 gut, the allantois, grows out. 



Before the end of the first 

 day, blood vessels begin to be 

 developed in the extra-embryo- 

 nic region of the blastoderm. 

 These form the beginning of the 

 vitelline vessels, which are of 

 great importance in the early 

 stages of development, and have probably at first some respiratory im- 

 portance. As development proceeds, the allantois increases greatly, and, 

 fusing with the subzonal membrane, approaches close to the egg shell. 

 It has a large blood supply, and functions as an organ of respiration, in 

 addition it absorbs the white of egg, thus serving as an organ of nutri- 

 tion, and also receives deposits of urates, thus functioning as an organ 

 of excretion in the narrow sense of the term. 



We have spoken of the "folding off" of the embryo ; it is important 



FIG. 219. Diagrammatic section 

 of embryo within Egg. (After 

 KENNEL. ) 



Z>., Yolk sac ; da., gut of embryo ; /., 

 /., inner and outer wall of the allantois ; 

 am., amnion proper ; a., within amniotic 

 cavity ; s., sub-zonal membrane. 



