THE EVIDENCE FROM DEVELOPMENT. 181 



region, so two folds grow downwards, and thus tend to form the body- 

 walls of the young animal. Contemporaneously with these changes 

 we find a structure of high importance to be gradually formed in the 

 back region of the chick. This structure is the " notochord," a rod- 

 like body (Fig. 94, cfi), composed of a string of cells, which lies just 

 beneath the first-formed tube (c), or that containing the nervous 

 rudiments. The formation of this notochord cannot but forcibly call 

 to mind the similar string of cells which appears in the course of 

 development in the sea-squirt's larva ; such a similarity being of too 

 marked a character to admit of its being regarded in the light of a 

 mere coincidence. On each side of the notochord the elements 

 of the spine, in the shape of little cubical vertebrae (Figs. 95, c, e; 

 93, F, /; and 96, pv\ are duly formed from the middle layer or 

 mesoblast. The notochord itself a permanent structure in such 

 fishes as the lancelet, sharks, dog-fishes, &c. gradually disappears in 

 the chick ; its retrogression being apparent after the sixth day, whilst 

 it is found to have entirely disappeared at the time of hatching ; 

 whatever of its substance remains being absorbed in the formation 

 of the spine. The folding of the blastoderm in front and behind 

 soon specialises the head and tail (Fig. 96, f) of the young animal ; 

 the head extremity presently showing three swellings (fb, mb, hb), 

 or dilatations from which the brain is duly formed and bending 

 downwards in a highly characteristic fashion. Brain-development is 

 accompanied by the formation of organs of sense, such as the nose, 

 eyes (op v], and ears (au /), which arise as pocket-like ingrowths from 

 the epiblast or outer layer of the body ; whilst the mouth is similarly 

 formed by an infolding of the outer layer, and is later on placed in 

 communication with the digestive system. 



During the third day of incubation, certain highly important 

 structures appear in the neck of the chick. Four clefts or slits 

 (Fig. 97, A, g) are formed in the walls of the throat, these being named 

 the visceral or branchial clefts. The 

 upper edges of the clefts form thick 

 folds, named the branchial 'folds \ five 

 folds existing to the four clefts, as 

 the last cleft has its lower border 

 thickened in addition to its upper 

 edge. The significance of these 

 clefts and folds will be hereafter 

 alluded to ; it may at present, how- 

 ever, be noted that all the clefts in 



the chick Save the first are Closed FIG. 97. EMBRYO-VERTEBRATES. 



by the seventh day of incubation. 



The visceral "folds" contribute in an important fashion to the 



formation of the jaws and other structures belonging to the skull, 



