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TEXT-BOOK OF EMBRYOLOGY. 



blastoderm. Concomitant with involution there is a considerable thick- 

 ening of the lip of the blastopore where the ectoderm and entoderm are 

 continuous. 



^ It has been noted that the germ wall is interrupted along the posterior 

 margin of the sector after the disk has here been reduced to one layer of cells. 

 The margin of the sector is obviously a crescent, so that the blastopore also 

 is originally crescent-shaped (Fig. 43, A). Then as gastrulation proceeds 

 the horns of the crescent are withdrawn toward the median line, and concomi- 



FIG. 43. Diagrammatic reconstructions showing surface views of blastoderms of the pigeon. 

 Patterson, from Lillie. A , from same blastoderm as shown in Fig. 41 , the line CD indicating 

 the plane of section of Fig. 41; the numbers 1-7 indicate the thickness of the blastoderm in 

 numbers of cells; the broken line around i includes the sector which is one cell thick, at the 

 posterior margin of which invagination begins; GW, germ wall. B, from same blastoderm 

 as shown in Fig. 42; the arrows at the posterior margin indicate the advance and approach 

 of the two halves of the margin; E, indicates extent of entoderm; O, extension of disk mar- 

 gin beyond germ wall; PA, outer margin of area pellucida; R, margin where invagination is 

 progressing (lip of blastopore) ; Y and Z together indicate region of germ wall. C, from a 

 blastoderm of pigeon 38 hours after fertilization; E indicates extent of entoderm; R, mass 

 of cells where blastopore closed; SG, portion of blastoccel not yet crossed by migrating 

 entodermal cells; other abbreviations as in B. 



tantly the two free ends of the germ wall approach each other (Fig. 43, B). 

 Eventually the ends of the germ wall meet and the blastopore is closed ; and 

 since the germ wall lies behind the closed blastopore, the latter is no longer 

 situated on the edge of the disk but is included within it (Fig. 43, C). 



