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In Selachian embryos an intermediate cell-mass, or middle plate of mesoblast is formed, as in birds, from a partial fusion of the somatic and splanchnic layers of the mesoblast at the outer border of the protovertebræ. From this cell-mass the whole of the urinogenital system is developed.
At about the time when three visceral clefts have appeared, there arises from the intermediate cell-mass, opposite the fifth protovertebra, a solid knob, from which a column of cells grows backwards to opposite the position of the future anus (Fig. 1. pd.).
Fig. 1. Two sections of a Pristiurus Embryo with three visceral clefts.
The sections are to shew the development of the segmental duct (pd) or primitive duct of the kidneys. In A (the anterior of the two sections) this appears as a solid knob projecting towards the epiblast. In B is seen a section of the column which has grown backwards from the knob in A.
spn. rudiment of a spinal nerve; mc. medullary canal; ch. notochord; X. string of cells below the notochord; mp. muscle-plate; mp´. specially developed portion of muscle-plate; ao. dorsal aorta; pd. segmental duct; so. somatopleura; sp. splanchnopleura; pp. pleuro-peritoneal or body-cavity; ep. epiblast; al. alimentary canal.
This knob projects outwards toward the epiblast, and the column lies at first between the mesoblast and epiblast. The knob and column do not long remain solid. The knob becoming hollow acquires a wide opening into the pleuro-peritoneal or body-cavity, and the column a lumen; so that by the time that five visceral clefts have appeared, the two together form a duct closed behind, but communicating in front by a wide opening with the pleuro-peritoneal cavity.
Before these changes are accomplished, a series of solid[29] outgrowths of elements of the 'intermediate cell-mass' appear at the uppermost corner of the body-cavity. These soon become hollow and appear as involutions from the body-cavity, curling round the inner and dorsal side of the previously formed duct.
One involution of this kind makes its appearance for each protovertebra, and the first belongs to the protovertebra immediately behind the anterior end of the duct whose development has just been described. In Pristiurus there are in all 29 of these at this period. The last two or three arise from that portion of the body-cavity, which at this stage still exists behind the anus. The first-formed duct and the subsequent involutions are the rudiments of the whole of the urinary system. The duct is the primitive duct of the kidney[30]; I shall call it in future the segmental duct; and the involutions are the commencements of the segmental tubes which constitute the body of the kidney. I shall call them in future segmental tubes.