The tubules of the Wolffian body are derived from the intermediate cell mass, shewn in [fig. 401], between the upper end of the body cavity and the muscle-plate. In the Chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment the intermediate cell mass becomes detached from the peritoneal epithelium at certain points, remaining attached to it at other points, there being several such to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords ([fig. 402], st) which soon unite with the segmental duct (wd). Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel. In the Duck the attachment of the intermediate cell mass to the peritoneal epithelium is prolonged further back than in the Chick.
In the foremost segmental tubes, which never reach a very complete development, the peritoneal funnels widen considerably, while at the same time they acquire a distinct lumen. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity ([fig. 403], gl). There is thus formed a series of free peritoneal glomeruli belonging to the anterior Wolffian tubuli[260]. These tubuli become however early aborted.
In the case of the remaining tubules developed from the S-shaped cords the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the segmental duct. Their blind extremities constitute the rudiments of Malpighian bodies.
Fig. 402. Transverse section through the trunk of a Duck embryo with about twenty-four mesoblastic somites.
am. amnion; so. somatopleure; sp. splanchnopleure; wd. Wolffian duct; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord; ch. notochord; ao. aorta; hy. hypoblast.
In the posterior part of the Wolffian body of the Chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks up into oval vesicles similar to those of the Reptilia, which form the rudiments of the segmental tubes.
Fig. 403. Section through the external glomerulus of one of the anterior segmental tubes of an embryo Chick of about 100 h.
gl. glomerulus; ge. peritoneal epithelium; Wd. Wolffian duct; ao. aorta; me. mesentery. The segmental tube, and the connection between the external and internal parts of the glomerulus are not shewn in this figure.