358 THE EVOLUTION OF MAN. 



originate in the very simplest form. The organ of smell, 

 or the nose, appears quite in the front of the head, in the 

 shape of two little pits above the mouth-opening (Fig. 

 117, n). The organ of sight, or eye, also in the form of 

 a pit (Fig. 117, I, 118, h), comes next, behind the organ of 

 smell, towards which a considerable vesicular outgrowth of 

 the fore-brain grows on both sides of the head (Fig. 105, a). 

 Further back appears a third pit on each side of the head, 

 the first rudiment of the organ of hearing (Fig. 117, g). 

 No trace of the very marvellous future structure of these 

 organs, or of the characteristic form of the face, is yet to 

 be seen. 



The human embryo, having reached this stage of develop- 

 ment, is yet hardly distinguishable from the germ of any 

 of the higher Vertebrates. (Cf Plates I., VL, and YIL) All 

 the essential portions of the body are now begun : the head 

 with its primitive skull, the rudiments of the three higher 

 sense-organs, and the five brain-bladders, and the gill-arches 

 and gill-openings ; the trunk with the medulla, the rudi- 

 ments of the vertebral column, the chain of metamera, the 

 heart and principal blood-vessels, and, finally, the primitive 

 kidneys. Man, in this germ-stage, is a higher Vertebrate, and 

 yet there is no essential, morphological difference between 

 the human embryo and that of Mammals, Birds, Reptiles, 

 etc. (Plates VI. and VII., upper line ot sections). This is an 

 ontogenetic fact of the highest significance; from it are 

 drawn the most important phylogenetic conclusions. 



There is, however, as yet no trace of limbs. Though 

 the head and the trunk are already separated, though all 

 the important inner organs are begun, there is as yet no 

 trace of the limbs, or extremities, in this stage. These do 



