OUK EMBRYONIC DEVELOPMENT. 65 



That applies both to the bodily and to the mental 

 characteristics. 



The formation of the germinal layers by the repeated 

 division of the stem-cell, the growth of the gastrula 

 and of the later germ-structures which succeed it, take 

 place in man in just the same manner as in the other 

 higher mammals, under the peculiar conditions which 

 differentiate this group from the lower vertebrates. 

 In the earlier stages of development these special 

 characters of the placentalia are not to be detected. 

 The significant embryonic or larvel form of the 

 chordula, which succeeds the gastrula, has sub- 

 stantially the same structure in all vertebrates ; a 

 simple straight rod, the dorsal cord, lies lengthways 

 along the main axis of the shield- shaped body — the 

 "embryonic shield"; above the cord the spinal 

 marrow developes out of the outer germinal layer, 

 while the gut makes its appearance underneath. 

 Then, on both sides, to the right and left of the axial 

 rod, appear the segments of the " pro-vertebrae " and 

 the outlines of the muscular plates, with which the 

 formation of the members of the vertebrate body 

 begins. The gill-clefts appear on either side of the 

 fore-gut ; they are the openings of the gullet, through 

 which, in our primitive fish-ancestors, the water which 

 had entered at the mouth for breathing purposes made 

 its exit at the sides of the head. By a tenacious 

 heredity these gill-clefts, which have no meaning 

 except for our fish-like aquatic ancestors, are still 

 preserved in the embryo of man and all the other 

 vertebrates. They disappear after a time. Even after 

 the five vesicles of the embryonic brain appear in the 

 head, and the rudiments of the eyes and ears at the 

 sides, and after the legs sprout out at the base of the 



