580 



GASTRULA-STAGE 



In the earthworm the blastopore does not become closed, but gives 

 rise to the mouth. 



In the frog (p. 201) the archenteron arises by a split appearing 

 amongst the yolk-cells, beginning at the edge of the blastopore 

 and gradually extending forwards : the process is probably supplemented 

 by a limited amount of invagination of the ectoderm. The archenteron 

 is at first a very narrow cleft, but soon widens considerably (Fig. 64, 

 F, ent] : for some time it does not actually communicate with the ex- 

 terior, the blastopore (blp] being filled up by a yolk-plug (yk. pi}. 



hd 



a-r.op 



pr.. 



mes 



FIG. 155. Two stages in the development of the blastoderm of the chick, at about 



the twentieth and twenty-fourth hour of incubation respectively ; diagrammatic. 

 ar. op. area opaca ; ar.pl. area pellucida ; hd. head ; ined.gr. medullary groove ; 

 mes. mesoderm, indicated by dotted outline and deeper shade ; pr. am. pro- 

 amnion ; pr. st. primitive streak and groove : pr. v. mesodermal segments or 

 " protovertebrae." (From Marshall's Embryology, in part after Duval.) 



As the archenteron extends forwards, and the relatively small segmen- 

 tation-cavity (bl. c&l\ gradually disappears, the edges of the lower margin 

 of the blastopore approach one another, and uniting in the median 

 plane, give rise to a vertical groove the primitive groove, as it is 

 called. 



In the centrolecithal egg of the crayfish (Fig. 98) a gastrula-stage is 

 formed by invagination, but as the centre of the oosperm is filled with 

 solid yolk in the place of a segmentation-cavity containing fluid, the 

 invagination only extends a short distance inwards, the archenteron 



