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 (i.e., 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 N.C. (G) New caudal opening is 

 forming, aided by proctodaeal invagination, PR.; tail rudiment elevation is indicated. 

 (H) Proctodaeal opening and tail rudiment are shown. 



