. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 416 GASTRULATION gresses. This particularly is true when the blastopore gradually grows smaller toward the end of the gastrulative process (Lewis, '49). 2) Epiboly. Intimately associated with and aiding the above processes in- volved in emboly are the movements concerned with epiboly. These move- ments result from cell proliferation, associated with a marked antero-posterior extension and expansion of the presumptive epidermal and neural plate are
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 416 GASTRULATION gresses. This particularly is true when the blastopore gradually grows smaller toward the end of the gastrulative process (Lewis, '49). 2) Epiboly. Intimately associated with and aiding the above processes in- volved in emboly are the movements concerned with epiboly. These move- ments result from cell proliferation, associated with a marked antero-posterior extension and expansion of the presumptive epidermal and neural plate areas. These changes are integrated closely with the inward migration of cells of the marginal zone (, chordamesoderm), and the presumptive epidermal and neural areas approach closer and closer to the blastoporal edge, until finally, when mesodermal and notochordal cells have entirely involuted, ecto- dermal cells occupy the rim of the blastopore as it closes (figs. 192A; 220D).. Fig. 197. History of the blastopore and adjacent posterior areas of developing embryo of the frog, Rana pipiens. (A) Dorsal lip of blastopore. Arrows show direction of initial invagination to form the dorsal lip. (B) Dorso-lateral and lateral-lip portions of the blastopore are added to original dorsal-lip area by convergence of mesodermal cells (arrows) and their involution at the edge of the lip. Entodermal material is invagi- nating. (C) Blastopore is complete; yolk plug is showing. (D) Toward the end of gastrulation. Blastopore is small; neural plate area becomes evident as neural folds begin their elevation. (E) Neural folds are slightly elevated; blastopore is very small; size of blastopore at this time is quite variable. (F) Blastopore has closed; neural folds are well developed; neurenteric passageway between neural folds and dorsal evagination of archenteric space into tail-bud area is indicated by (G) New caudal opening is forming, aided by proctodaeal invagination, PR.; tail rudiment el
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