Fig. 106. Diagrammatic longitudinal section through the posterior end of an Embryo Bird at the time of the formation of the allantois.
ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. neurenteric canal; hy. hypoblast; p.a.g. postanal gut; pr. remains of primitive streak folded in on the ventral side; al. allantois; me. mesoblast; an. point where anus will be formed; p.c. perivisceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure.
The hypoblast. The closure of the alimentary canal is entirely effected by a process of tucking in or folding off of the embryo from the yolk-sack. The general nature of the process is seen in the diagrams [figs. 105] and [121]. The folds by which it is effected are usually distinguished as the head-, the tail- and the lateral folds. The head-fold ([fig. 105]) is the first to appear; and in combination with the lateral folds gives rise to the anterior part of the mesenteron (D) (including the œsophagus, stomach and duodenum), which by its mode of formation clearly ends blindly in front. The tail-fold, in combination with the two lateral folds, gives rise to the hinder part of the alimentary tract, including the cloaca, which is a true part of the mesenteron. At the junction between the two folds there is present a circular opening leading into the yolk-sack, which becomes gradually narrowed as development proceeds. The opening is completely closed long before the embryo is hatched. Certain peculiarities in reference to the structure of the tail-fold are caused by the formation of the allantois, and are described with the embryonic appendages. The stomodæum and proctodæum are formed by epiblastic invaginations. The communication between the stomodæum and the mesenteron is effected comparatively early (on the 4th day in the chick), while that between the proctodæum and mesenteron does not take place till very late (15th day in the chick). The proctodæum gives rise to the bursa Fabricii, as well as to the anus. Although the opening of the anus is so late in being formed, the proctodæum itself is very early apparent. Soon after the hinder part of the primitive streak becomes tucked in on the ventral side of the embryo, an invagination may be noticed where the tail of the embryo is folded off. This gradually becomes deeper, and finally comes into contact with the hypoblast at the front (primitively the apparent hind) border of the posterior section of the primitive streak. An early stage in the invagination is shewn in the diagram ([fig. 106], an). It deserves to be noted that the anus lies some way in front of the blind end of the mesenteron, so that there is in fact a well-developed postanal 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 postanal gut with the medullary tube in the duck, fowl, and other birds. On the ventral wall of the postanal 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 postanal 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.
Fig. 107. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.
a.pr. anterior part of the primitive streak; p.pr. posterior part of the primitive streak.
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.
