266 THE EVOLUTION OF MAN. 



fcine, along the central line on the side toward the abdomen, 

 and conveys carbonated, or venous blood, from the body- 

 back to the gills. In the front part of the gill-division of 

 the intestine, these two main channels are connected by- 

 several connecting branches, which rise in the form of 

 arches between the gill-openings. These "vascular gill- 

 arches" (kg) pass along the gill-openings, and directly 

 accomplish respiration. Immediately behind their base the 

 front end of the primitive vein enlarges into a spindle-shaped 

 bladder Qiz). This is the simplest rudiment of the heart, 

 which, in higher Vertebrates and in Man, afterwards as- 

 sumes the form of a four-chambered, pulsating organ. 



In the lowest part of the body-cavity of Vertebrates, 

 on the under side of the dorsal wall, near and on both sides 

 of the notochord and the mesentery, lie the sexual glands, 

 which form the reproductive cells ; in the female the ovary, 

 in the male the testis. Recent study of the development 

 of these parts seems to show that the original formation 

 of the sexual glands in mankind and in all other Verte- 

 brates, is hermaphroditic, or sexless. The embryonic glands 

 of the Vertebrate contain the rudiments of both kinds of 

 reproductive organs — the ovary of the female, which forms 

 fche ovule; and the testis of the male, which forms the 

 sperm. These two kinds of sexual glands, each of which at 

 a later stage of development is distributed to one only of 

 the two sexes, are originally united in the embryo. This 

 fact leads us to the conviction, which appears probable on 

 other grounds also, that Vertebrates, in common with lower 

 animals, were originally hermaphrodite, that each indi- 

 vidual was capable of reproducing itself independently, and 

 that the separation of the sexual organs took place at a 



