566 



COLLEGE ZOOLOGY 



sequently is completely bounded by this 

 germ layer; this cavity is the coelom (Fig. 

 402). The outer layer of mesoderm adjacent 

 to the ectoderm is now called the somatic 

 mesoderm; together with the ectoderm it 

 forms the body wall. The inner layer of 

 mesoderm adjacent to the endoderm is 

 now called splanchnic mesoderm; together 

 with the endoderm it forms the wall of the 

 digestive tube. If the developing egg belongs 

 to a chordate, coelom formation does not 

 extend into the dorsal-most mesoderm. The 

 mesoderm located in the dorsal midline 

 forms a longitudinal supporting rod, the 

 notochord. The mesoderm to each side of 

 the latter becomes separated into a row of 

 mesodermal blocks, the somites. The ar- 

 rangement of these pairs of somites along 

 the length of the embryo is the basis of 

 metamerism in the chordate body. 



A most interesting change then occurs in 

 the ectoderm located directly above the 

 notochord and somites (Fig. 402). It be- 

 comes thickened into a flat plate of cells, 

 the neural plate; the edges of this plate are 

 then elevated as neural folds. These folds 

 continue to approach one another until they 

 fuse in the middorsal line, forming a dorsal, 

 hollow, neural tube, covered by ectoderm 

 (Fig. 402). Once this developmental stage 

 is reached (the neurula), subsequent de- 

 velopmental processes are relatively simple, 

 although they result eventually in the forma- 

 tion of complex organs. The anterior end of 

 the neural tube enlarges to become the 

 brain; different antero-posterior levels of the 

 brain enlarge to different degrees, thereby 

 indicating the 5 major subdivisions of the 

 adult brain. The lateral walls of the second 

 such subdivision fold outwards (normal em- 

 bryo, right side of Fig. 403), then fold back 

 upon themselves (normal embryo, left side 

 of Fig. 403), thereby forming the major 

 part of the eye, the optic cup. The ecto- 

 derm overlying this optic cup thickens, then 

 folds towards the optic cup (right side of 

 Fig. 403), forms a vesicle, and finally sepa- 

 rates from the ectoderm to become the lens 



of the eye (left side of Fig. 403). At the an- 

 terior end of the brain a pair of infoldings 

 of the ectoderm marks the beginnings of the 

 nostrils. On each side of the hindbrain a 

 similar infolding of the ectoderm forms an 

 auditory vesicle which separates from the 

 ectoderm; each of these vesicles later on 

 transforms into the complex inner ear mech- 

 anism (membranous labyrinth). The pos- 

 terior end of the neural tube remains less 

 complex and becomes the spinal cord. 



Similarly, the digestive tube becomes 

 modified at different antero-posterior levels 

 to form the posterior part of the mouth, 

 pharynx, esophagus, stomach, and intes- 

 tine. At certain levels of this digestive tube 

 the walls fold outwards to form several de- 

 rivatives of the digestive tube: the trachea 

 and lungs, the common bile duct, liver, gall 

 bladder, and pancreas. 



At appropriate levels of the body, the 

 lateral body wall thickens and protrudes out- 

 ward to form the limb buds (Fig. 402), 

 which develop later into the legs. Certain 

 areas of the mesoderm form the heart, 

 others the kidney tubules, and still others 

 the gonads (Fig. 402). 



Thus as development continues beyond 

 the gastrula stage, different groups of cells, 

 which originated by cleavage of the fertilized 

 egg, and which accordingly contain parts of 

 the original cytoplasm of the egg and the 

 same chromosomes as the zygote, develop 

 in different directions; we speak of this 

 stage of development as the stage of cellular 

 differentiation. The question then arises, 

 why do cells become different from one 

 another during development? Descriptive 

 embryologists suspected that interaction be- 

 tween adjacent groups of cells might play 

 a role in cellular differentiation. For exam- 

 ple, they could see that the lens always de- 

 veloped from the ectoderm immediately ad- 

 jacent to the optic cup. Perhaps the optic 

 cup exerts some influence on the overlying 

 ectoderm which causes it to differentiate 

 into a lens. How could this suspicion (hypo- 

 thesis) be tested? 



