During the important processes which lead to the formation of the navel, and of the intestinal wall and ventral wall, we find a number of other interesting changes taking place in the embryonic shield of the amniotes. These relate chiefly to the prorenal ducts and the first blood-vessels. The prorenal (primitive kidney) ducts, which at first lie quite flat under the horn-plate or epiderm (Figure 1.93 ung), soon back towards each other in consequence of special growth movements (Figures 1.143 to 1.145 ung). They depart more and more from their point of origin, and approach the gut-gland layer. In the end they lie deep in the interior, on either side of the mesentery, underneath the chorda, (Figure 1.145 ung). At the same time, the two primitive aortas change their position (cf. Figures 1.138 to 1.145 ao); they travel inwards underneath the chorda, and there coalesce at last to form a single secondary aorta, which is found under the rudimentary vertebral column (Figure 1.145 ao). The cardinal veins, the first venous blood-vessels, also back towards each other, and eventually unite immediately above the rudimentary kidneys (Figures 1.145 vc, 152 cav). In the same spot, at the inner side of the fore-kidneys, we soon see the first trace of the sexual organs. The most important part of this apparatus (apart from all its appendages) is the ovary in the female and the testicle in the male. Both develop from a small part of the cell-lining of the body-cavity, at the spot where the skin-fibre layer and gut-fibre layer touch. The connection of this embryonic gland with the prorenal ducts, which lie close to it and assume most important relations to it, is only secondary. (FIGURE 1.149. Longitudinal section of a human embryo of the fourth week, one-fifth of an inch long, magnified fifteen times. Showing: bend of skull, yelk-sac, umbilical cord, terminal gut, rudimentary kidneys, mesoderm, head-gut (with gill-clefts), primitive lungs, liver, stomach, pancreas, mesentery, primitive kidneys, allantoic duct, rectum. (From Kollmann.) FIGURE 1.150. Transverse section of a human embryo of fourteen days. mr medullary tube, ch chorda. vu umbilical vein, mt myotome, mp middle plate, ug prorenal duct, lh body-cavity, e ectoderm, bh ventral skin, hf skin-fibre layer, df gut-fibre layer. (From Kollmann.) FIGURE 1.151. Transverse section of a shark-embryo (or young selachius). mr medullary tube, ch chorda, a aorta, d gut, vp principal (or subintestinal) vein, mt myotome, mm muscular mass of the provertebra, mp middle plate, ug prorenal duct, lh body-cavity, e ectoderm of the rudimentary extremities, mz mesenchymic cells, z point where the myotome and nephrotome separate. (From H.E. Ziegler.)