94 



COMPARATIVE ANATOMY 



forming. Shortly the somites become detached from the lower somatic 

 and visceral sheets of mesoderm and the myocoeles lose continuity with 

 the permanent coelom (Fig. 64B) . Eventually, as the somite differentiates, 

 the myocoele is obliterated. 



The differentiation of the mesoderm at the stage just described — 

 dorsally the pairs of quite detached hollow somites; ventrally the unseg- 

 mented somatic and visceral layers enclosing between them the coelom — 

 has its exact counterpart in Amphioxus. But in Amphioxus segmentation 



and hollowness of the mesoderm are 

 primary and continuity of the coelomic 

 space is secondarily acquired. In verte- 

 brates segmentation is secondary and so 

 isp also is hollowness except that in cyclo- 

 stomes and some urodele amphibians the 

 early mesoderm appears to have folded 

 outward from the dorsal wall of the 

 enteron and exhibits a thin cavity which 

 Fig. 66. — Stereogram of the may be continuous with the enteric cavity 



anterior region of an amphibian .p. ^^^ ^j^j^ exceptional Condition, 



(urodele) embryo, a, anterior end; \ b -> f 



ar, archenteroh; c, coelom; ch, noto- although it involves uo primary segmen- 



chordal cells; e, yolk cells; ec,ecto- . ^^ ^g ^he mode of Origin of the 



derm; mp, mesodermal pouch; ng, . . 



primitive groove; np, neural plate; mesoderm in AmphioxuS. 



nr, neural folds; sc, segmentation j • f ^^er history, however, the 



cavity (blastocoele) ; so, somatic wall -' ' 



of coelom; 5;^, splanchnic (visceral) differentiation of the vertebrate mesoderm 

 wall of coelom. (From Kingsiey ) .^ ^^^^ elaborate than that of Amphioxus, 



especially in the prospective trunk region. Here a region of mesoderm 

 intermediate between the upper somite or muscle-forming part and the 

 lower peritoneum-forming layers becomes early designated to play a very 

 important role, the formation of the essential structures of the kidneys. 

 In the trunk mesoderm, then, there are upon each side three zones of 

 differentiation: the epimere, a dorsal muscle-forming part; the mesomere, 

 a kidney-forming zone situated just below the epimere; and the hypomere, 

 the most ventral zone, constituting the somatic and visceral layers of 

 peritoneum (Figs. 63 and 64^). 



The epimere undergoes three kinds of differentiation. Its heavier 

 inner wall is mainly converted into striated body-muscle. The embryonic 

 myotome gives rise not only to the dorsal but to the ventral body-muscle. 

 The myotome material grows ventral-wards, pushing its way between 

 the ectoderm and the somatic mesoderm, until it reaches the mid-ventral 

 plane (compare Figs. 67 and 68). From the medial surface of the inner 

 or myotomic wall of the epimere numerous cells become detached and 

 give rise to loosely aggregated cellular masses surrounding the notochord 

 and neural tube (Fig. 69). Most of this material enters into the forma- 



