444 



THE EVOLUTION OF MAN. 



At last, the inversion is complete, so that the inner part of 

 the wall, that which has been inverted, lies on the inside of 

 the outer, the uninverted part. In this way an almost 

 hemispherical hollow body is formed, the thin wall of which 

 is composed of two layers of cells. The hemispherical form 

 soon again changes into an almost spherical or oval shape, 

 in consequence of the inner cavity becoming considerably 

 enlarged, while its opening becomes narrower (Plate X. 

 Fig. 10). The form which the embryo of the Amphioxus 

 has now attained in this way is a true Gastrula or intes- 

 tinal larva ; is indeed a gastrula of that original and 

 simplest form which we have already distinguished as the 

 Bell-gastrula or Arcliigastrula (p. 191, Fig. 22,7, K). 



Fig. 155. — Gastrula of Amphi- 

 oxns, in longitudinal section : d, 

 primitive intestine ; o, primitive 

 mouth ; i, intestinal layer, or ento" 

 derm ; e, skin-layer, or exoderm. 



As in all those lowly 

 organized animals which 

 form a primitive Bell-gas- 

 trula of this sort, the body 

 of the Amphioxus, which 

 has but one axis, is merely 

 a simple intestinal pouch ; 

 the inner cavity of this is the primitive intestine (proto- 

 gaster) (Fig. 155, d, Fig. 156,^); its simple opening is the 

 primitive mouth {'protostomo.., o). The wall is at once 

 the intestinal wall and the body-wall. It is composed of 

 two cell- strata, of the two well-known primary germ-layers. 

 The inner stratum, or the inverted portion of the hlastula. 



