THE REPRODUCTION OF ANIMALS 239 



then folds, to form first a trough and then a tube that is cut off from the 

 remainder of the ectoderm and lies beneath the outer ectoderm of the 

 middorsal wall of the embryo, just above the notochord. 



The rapid multiplication of the cells of the neural tube continues, and 

 the tube soon develops a number of expansions that represent the early- 

 stages of the several brain regions. The longer portion of the tube posterior 

 to the brain becomes the spinal cord. 



The gill pouches. Another peculiar and universal feature of all verte- 

 brate embryos is the formation of several pairs of gill pouches and gill 

 arches. The side walls of the fore portion of the early embryonic gut, which 

 forms at a very early stage in embryonic development, soon produce a 

 series of paired (right and left) pouchlike outgrowths that extend toward 

 the body wall of the neck region. At a slightly later stage the outer body 

 wall begins to grow inward in a series of lateral furrows, each furrow 

 opposite the apex of an outwardly growing pouch. The partitions between 

 these paired pouches become regions of rapid mesodermal growth, and a 

 series of alternating gill pouches and gill arches is soon formed on either 

 side of the fore-gut. In the fishes and amphibians the matched pouches 

 and clefts soon unite (break through) to form gill slits, connecting the 

 cavity of the fore-gut with the outside environment. In these groups the 

 gill arches become the supporting structures on which the gill membranes 

 are developed and through which the blood vessels (aortic arches) run 

 that connect the dorsal and ventral aortas. In the higher vertebrates — 

 reptiles, birds, and mammals — the pouches and furrows do not completely 

 break through, but they and the arches between them do profoundly 



junction of the neural folds and anteriorly is developing rapidly into the primary brain 

 vesicles. The heart has also formed as a pulsing tube connected with the blood vessels 

 developing from the nearer blood islands in the extraembryonic region. Note the increased 

 number of somites. 



E, a cross section through D along the line indicated. The neural tube was formed by an 

 infolding and pinching off of the ectoderm; the inside of the tube was formerly the upper 

 surface of the ectodermal sheet along the midline. (Compare B and C.) 



F, the embryo after 48 hours of incubation, with the extraembryonic membranes re- 

 moved. The rapidly developing neural tube has curved and twisted through nearly 90 

 degrees, so that the embryo now lies on its side anteriorly. Note the developing eye and ear, 

 and the growing heart, which has elongated and looped. Note also the great development of 

 extraembryonic blood vessels. The embryonic blood vessels are not visible except where they 

 are somewhat indicated in the heart region. 



G, the embryo after 72 hours of incubation, with the extraembryonic membranes re- 

 moved. Note the appearance of limb buds and tail, and that the region above the heart 

 shows gill arches and furrows. 



H, the embryo after 96 hours of incubation. 



/, the embryo after 5 days of incubation. The embryo is now much more opaque, and only 

 surface features are shown. Note the great enlargement of the limb buds, and the appear- 

 ance of digits on the forelimbs. Sixteen more days of incubation are required before hatch- 

 ing, alb, anterior limb bud; aln, allantois; als, allantoic stalk; ao, aorta; aop, area opaca; apl, 

 area pellucida; art, artery; bdk, blastodisk; bit, blood islands; 6?;, blood vessels; elm, coelom; 

 ect, ectoderm; ent, entoderm; ga, gill arches ;od, gill clefts; ht, heart; liv, liver; mes, mesoderm; 

 nch, notochord; np, nasal pit; npl, neural plate; nt, neural tube; opl, optic lobes of brain; plb, 

 posterior limb bud; sh, shell; sow, somite; vn, vein; wh, white; y, yolk; ym, yolk membrane; 

 yst, yolk stalk. 



