4 INTRODUCTIOX 



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

 cleavage cavity or Von Baers cavity (hlastocoele), which con- 

 tinually increases during the succeeding cell divisions, while 

 the blastomeres arrange themselves about this cavity in a 

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

 ment thus reached is known as the hlastula or hlastosphere. 

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

 e^^ 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 ; whereas 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 wall of the single- 

 layer blastosphere represents the first of the primitive organs 

 of the Metazoan body. 



In the simplest cases a gastrula- 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 preserved only as 

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

 produced by the metamorphosis already described. The 

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

 encloses a cavity which has arisen by invagination, called 

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

 of the vegetative pole by means of the primitive mouth 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 blastosphere, 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 layers be- 

 come continuous with each other. Ectoderm and entoderm 



^ 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 Limax," Proc. Amer. 

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



