4 INTRODUCTION 



separation of the cells there soon arises a central cavity, the 

 cleavage cavity or Vmi Baers cavity {hlaf;toc<ele), which con- 

 tinually increases during the succeeding cell divisions, while 

 the blastomeres ari-ange themselves about this cavity in a 

 single-layer epithelium (blastoderm). The stage of develop- 

 ment thus reached is known as the hlastula or blastosphere. 

 In the one-layer hlastula an arrangement of the parts of the 

 egg about the chief axis is also clearly recognizable. The 

 cells in the vicinity of the animal pole are, as a rule, smaller 

 and not so rich in nutritive yolk particles ; Avhereas the cells 

 of the vegetative portion are larger and richer in yolk, and, 

 in consequence of the impeding influence offered by the 

 nutritive yolk, divide more slowly.^ The Avail of the single- 

 layer blastosphere represents the first of the primitive organs 

 of the Metazoan body. 



In the simplest cases a gastrvla-stage is developed out of 

 the blastula-stage by the cell-layer of the vegetative half 

 becoming flattened and gradually depressed, so that there 

 arises an ever-deepening invagination at the vegetative pole. 

 In this way the cleavage cavity (primitive body cavity) 

 becomes gradually reduced, and often is presex'ved only as 

 a narrow cleft between the two layers of the body-wall 

 produced by the metamorphosis already desci'ibed. The 

 gastrula-stage has substantially the form of a sac. Ifc 

 encloses a cavity which has arisen by invagination, called 

 the arche7iteric cavity, and opens to the exterior in the region 

 of the vegetative pole by means of the primitive month or 

 prostoma. (blastopore). The wall at this stage consists of two 

 cell-layers : an outer, the ectoderm, which is derived from the 

 cells of the animal portion of the blastosphei-e, and an inner, 

 the entoderm, which consists of the cells of the former 

 vegetative half, and which has reached the inside of the 

 embryo by the process of invagination. In the region of 

 the blastopore the ectodermal and entodermal la3^ers be- 

 come continuous with each other. Ectoderm and entoderm 



1 There are reasons for thinking that the rate of cleavage is not wholly 

 dependent on the proportion of nutritive yolk in the blastomere. (See 

 KoFoiD, C. A., " On Some Laws of Cleavage in Liniax," Proc. Amcr. 

 Acad. Arts and Sciences, vol. xxix., p. 180, 1894) [Translators]. 



