558 GASTRULA chak 



of the two resulting cells dividing again into two (C, D). 

 This process is continued until a globular mass of cells or 

 blastomeres is produced by the repeated division of the 

 one cell which forms the starting point of the series. 

 Owing to there teing rather more yolk at the lower than at 

 the upper pole, the lower cells are slightly larger than the 

 upper, so that the segmentation is not quite equal (E — -K). 

 The embryo has now arrived at the polyplast or morula 

 stage, and sections show that it is hollow, the blastomeres 

 being arranged in a single layer around a central segmenta 

 tion-cavity (p. 200) : such a hollow polyplast is often known 

 as a blastitla {K). The lower side of the blastula then 

 becomes tucked in, or invagitiated, the result being that the 

 single-layered sphere is converted into a double-layered cup 

 (Fig. 142). This process can be sufificiently well imitated 

 by pushing in one pole of a hollow india-rubber ball with 

 the finger. The resulting embryonic stage is known as the 

 gastrula (Fig. 142, C) : its cavity is the primitive enteron 

 or archenteron (p. 201), and is bounded by the invaginated 

 cells which now constitute the endoderm, the remaining cells 

 forming the outer wall of the gastrula being the ectoderm 

 (p. 202). The two layers are continuous at the aperture 

 of the cup, the gastrula-mouth or blastopore. Between the 

 ectoderm and endoderm is at first a space, the greatly dimi- 

 nished segmentation-cavity, which gradually becomes entirely 

 obliterated, so that the ectoderm and endoderm are in con- 

 tact (A.B). The general resemblance of the gastrula to a 

 simplified Hydra,^ devoid of tentacles, will at once be ap- 

 parent, and the stage in the development of the frog's egg re- 



' It must, however, be remembered (pp. 303 and 313) that the 

 ectoderm and endoderm of Hydroids are differentiated before the 

 mouth is formed, so that the mouth does not correspond to the blasto- 

 pore of the gastrula. 



