55^ GASTRULA thah. 



of the two resulting cells dividing again into two ((.', 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 being 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 

 tioii-cavifv (p. 200) : such a hollow polyplast is often known 

 as a hiasttila (A'). The lower side of the blastula then 

 becomes tucked in, or i/ivagina/ed, the result being that the 

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

 (l''ig. 142). This process can be sufficiently well imitated 

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

 the finger. The resulting embr)onic stage is known as the 

 Xi!sfr///a (Fig. 142, C) : its cavity is the primitive enteron 

 ox airheiiteroii (p. 201), and is bounded by the invaginated 

 cells which now constitute the oidoderin, the remaining cells 

 forming the outer wall of the gastrula being the ectode7-m 

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

 of the cup, the gastnila-iiwiil]i or hlastflpore. 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 arc 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 fi'og's egg re- 



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

 ectoderm and endoderm of Ilydroids are differentiated before the 

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

 pore of tlte gastrula. 



