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 (Fig. 93 _ung_), soon back towards each other in consequence of special growth movements (Figs. 143–145 _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, (Fig. 145 _ung_). At the same time, the two primitive aortas change their position (cf. Figs. 138–145 _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 (Fig. 145 _ao_). The cardinal veins, the first venous blood-vessels, also back towards each other, and eventually unite immediately above the rudimentary kidneys (Figs. 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. Fig.149. Longitudinal section of a human embryo of the fourth week, one-fifth of an inch long. Fig. 149—Longitudinal section of a human embryo of the fourth week, one-fifth of an inch long, magnified. (From _Kollmann._) Fig.150. Transverse section of a human embryo of fourteen days. Fig. 151. Transverse section of a shark-embryo (or young selachius). Fig. 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._) Fig. 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._)