MODIFICATIONS OF THE VASCULAR SYSTEM. 34! 



Similarly the two primitive aortse penetrate within, 

 below the notochord, and there eventually amalgamate 

 and form a single secondary aorta, which is situated under 

 the rudimentary vertebral column. (Cf. Figs. 95-98, ao.) 

 So, too, the cardinal veins, the first rudiments of the 

 venous blood-vessels, penetrate further inwards, and after- 

 wards lie directly over the primitive kidneys (Fig. 97, vc). 

 In the same locality, at the inner side of the primitive 

 kidneys, the first rudiments of the sexual organs soon 

 become visible. The chief portion of this apparatus, apart 

 from all its appendages, is, in the female, the ovary in the 

 male, the testes. Originally both these appear in the form 

 of a simple hermaphrodite gland, formed from a small por- 

 tion of the co3lom-epithelium, the cellular lining of the body- 

 cavity, at the point of contact between the skin-fibrous layer 

 and the intestinal-fibrous layer. It is only secondarily that 

 this hermaphrodite gland seems to connect itself with the 

 primitive kidney ducts, which lie very close to them, and 

 which are very importantly related to the sexual glands. 

 (Cf. Chap. XXV., and Plate IV. Figs. 5-7.) 



We will now leave the transverse sections of the verte- 

 brate body, the comparison of which has been so ex- 

 tremely instructive and important, and by means of which 

 we have solved the hardest problem of germ-history, the 

 problem as to the part taken by the germ-layers in the 

 formation of the body. In place of those, we will now 

 examine the longitudinal form of the rudimentary embryo 

 of the mammalian body, partly superficially, and partly in 

 various longitudinal sections. 



Let us now examine the surface, from the dorsal side, of 

 that very simple embryonic form which we called the sole- 



