404 THE EVOLUTION OF MAN. 



Eighth Stage : Cyclostoma Stage (Fig. 132, p. 377, PI. VII. Fig. M I.). 



The human germ possesses, in essential points, the organization of a gill- 

 less cranial animal (like the developed Myxinoida and Petromyzonta). The 

 number of metamera is increasing. The head is more distinctly differenti- 

 ated from the trunk. The anterior extremity of the medullary tube swells 

 in the form of a bladder, and forms the rudimentary brain, which soon 

 divides into five brain-bladders, lying one behind the other. On the sides of 

 these appear the rudiments of the three higher sense-organs : the nose-pit, 

 and the eye and ear vesicles. With the first circulation of the blood the 

 heart begins its activity. The jaws and limbs are still wanting. 



Ninth Stage: Ichthyod Stage (Fig. 134, p. 378, PI. VII. Fig. if II.). 

 The human germ possesses, in essential points, the organization of a fish 

 (or a fish-like Skulled-animal) . The two pairs of limbs appear in the simplest 

 form, as fin-like processes : a pair of anterior limbs (dorsal fins) and one 

 pair of posterior limbs (ventral fins). The gill-openings are completely 

 formed, and between these the gill-arches form ; the first pair of gill-arches 

 differentiate into the rudiments of the upper and lower jaws. From the 

 intestinal canal proceed lungs (swimming-bladder), liver, and pancreas. 



Tenth Stage : Amniotic Stage (PL VII. Fig. If III. ; PI. VIII.). 

 The human germ possesses, in essential points, the organization of an 

 Amnion-animal (of a higher gill-less Vertebrate). The gill-openings disap- 

 pear by concrescence. From the gill-arches develop the jaws, the tongue- 

 bone, and the bonelets (ossicles) of the ear. The allantois perfects itself, 

 and changes into the peripheric portion of the placenta. All the organs 

 gradually acquire the forms peculiar to the mammals, and at last the 

 specific human form. (Compare on these points the Phylogeny in the 

 following chapters. 109 ) 



