The proctodæum very early opens into the mesenteron, but the stomodæum remains closed till the differentiation of the mid-gut is nearly completed. The proctodæum gives rise to the abdominal part of the intestine, and the stomodæum to the œsophagus and stomach. The commencement of the masticatory apparatus in the latter appears very early as a dorsal thickening of the epithelium.
The primitive mesenteron in Palæmon differentiates itself into the permanent mid-gut and liver in a manner generally similar to that in Astacus, though the process is considerably less complicated. A distinct layer of cells separates itself from the outer part of the yolk pyramids, and gives rise to the glandular lining both of the mid-gut and of the liver. The differentiation of this layer commences behind, and the mid-gut very soon communicates freely with the proctodæum. The lateral parts of the primitive mesenteron become constricted into four wings, two directed forwards and two backwards; these, after the yolk in them has become absorbed, constitute the liver. The median part simply becomes the mesenteron. The stomachic end of the stomodæum lies in contact with the mesenteron close to the point where it is continued into the hepatic diverticula, and, though the partition wall between the two becomes early very thin, a free communication is not established till the yolk has been completely absorbed.
The alimentary tract in the Isopoda is mainly if not entirely formed from the proctodæum and stomodæum, both of which arise before any other part of the alimentary system as epiblastic invaginations, and gradually grow inwards ([fig. 244]). In Oniscus the liver is formed as two discs at the surface of the yolk on each side of the anterior part of the body. Their walls are composed of cubical cells derived from the yolk cells, the origin of which was spoken of on p. [516]. These two discs gradually take the form of sacks ([fig. 244] B, li.) freely open on their inner side to the yolk. As these sacks continue to grow the stomodæum and proctodæum do not remain passive. The stomodæum, which gives rise to the œsophagus and stomach of the adult, soon exhibits a posterior dilatation destined to become the stomach, on the dorsal wall of which a well-marked prominence—the earliest trace of the future armature—is soon formed ([fig. 244] B, zp). The proctodæum (pr) grows with much greater rapidity than the stomodæum, and its end adjoining the yolk becomes extremely thin or even broken through. In the earliest stages it was surrounded by the yolk cells, but in its later growth the yolk cells become gradually reduced in number and appear to recede before it—so much so that one is led to conclude that the later growth of the proctodæum takes place at the expense of the yolk cells.
Fig. 244. Two longitudinal sections through the embryo of Oniscus murarius. (After Bobretzky.)
st. stomodæum; pr. proctodæum; hy. hypoblast formed of large nucleated cells imbedded in yolk; m. mesoblast; vg. ventral nerve cord; sg. supra-œsophageal ganglion; li. liver; do. dorsal organ; zp. rudiment of masticatory apparatus.
The liver sacks become filled with a granular material without a trace of cells; their posterior wall is continuous with the yolk cells, and their anterior lies close behind the stomach. The proctodæum continually grows forwards till it approaches close to the stomodæum, and the two liver sacks, now united into one at their base, become directly continuous with the proctodæum. By the stage when this junction is effected the yolk cells have completely disappeared. It seems then that in Oniscus the yolk cells (hypoblast) are mainly employed in giving rise to the walls of the liver; but that they probably also supply the material for the later growth of the apparent proctodæum. It becomes therefore necessary to conclude that the latter, which might seem, together with the stomodæum, to form the whole alimentary tract, does in reality correspond to the proctodæum and mesenteron together, though the digestive fluids are no doubt mainly secreted not in the mesenteron but in the hepatic diverticula. The proctodæum and stomodæum at first meet each other without communicating, but before long the partition between the two is broken through.
In Cymothoa (Bullar, No. [499]) the proctodæum and stomodæum develop in the same manner as in Oniscus, but the hypoblast has quite a different form. The main mass of the yolk, which is much greater than in Oniscus, is not contained in definite yolk cells, but the hypoblast is represented by (1) two solid masses of cells, derived apparently from the inner layer of blastoderm cells, which give rise to the liver; and (2) by a membrane enclosing the yolk in which nuclei are present.
The two hepatic masses lie on the surface of the yolk, and each of them becomes divided into three short cæcal tubes freely open to the yolk. The stomodæum soon reaches its full length, but the proctodæum grows forwards above the yolk till it meets the stomodæum. By the time this takes place the liver cæca have grown into three large tubes filled with fluid, and provided with a muscular wall. They now lie above the yolk, and no longer communicate directly with the cavity of the yolk-sack, but open together with the yolk-sack into the point of junction of the proctodæum and stomodæum. The yolk-sack of Cymothoa no doubt represents part of the mesenteron, but there is no evidence in favour of any part of the apparent proctodæum representing it also, though it is quite possible that it may do so. The relations of the yolk-sack and hepatic diverticula in Cymothoa appear to hold good for Asellus and probably for most Isopoda.