KOFOID: DEVELOPMENT OF LIMAX. 99 



limite entre les spherules apparait commc tine ligne d'epaisseur tres 

 appreciable et dout la transparence contraste aveo la tcinte foncoV des 

 cellules ; c'est la coupe optique de la couche de sarcode qui regne sur 

 toute la surface des spherules. Mais en outre nous remarquons dims le 

 plan de contact des cellules voisin.es une accumulation de liquide, fait 

 deja constate par YVaineck. Ce liquide va sans doute remplir la cavity 

 de fractionnement, mais une partie est aussi expulsee sous forme de 

 gouttelettes qui sortent generalment au pole nutritif. J'ai observe une 

 seule goutte chez les Heteropodes et les Pulmones aquatiques. ... La 

 cavite de segmentation, ou blastocele, reste presque nulle pendant que 

 l'invagination se forme et ne commence a croitre qu'apres le retrecisse- 

 ment du blastopore." 



The propriety of the use of the term blastocoel or segmentation cavity 

 to designate the lacunar spaces of the mesenchyma of the gastrula, as 

 well as the spaces between the primitive blastomeres, is questionable. 

 To be sure the mesenchymatous lacunae are derived from the blastocoel 

 rather than from an enterocoel, in Lirnax at least ; but our choice of 

 terms is not limited to blastocoel and enterocoel, and it seems preferable 

 to apply to those spaces without epithelial lining which lie between the 

 ectoderm and entoderm, and are traversed by loose mesenchymatous 

 cells and prolongations of the ectoderm cells, a term not already set 

 apart for another use. Previous to the formation of these lacunae, all 

 the cells of the intermediate layer exist as a solid mass obliterating 

 the cleavage cavity. When, however, in Limax the cavity reappears, 

 as in Figure 54, it is as a distinct space bounded by germ layers. It 

 seems better, therefore, to apply to the spaces mentioned in the sec- 

 ond quotation from Fol (p. 116) the same term which is used elsewhere 

 for mesenchymatous lacunae not lined by a distinct epithelial layer, 

 viz. schizocoel. 



Eabl ('79, p. 568) notes the presence of a cleavage cavity in the 

 twelve-cell stage of Planorbis, and suggests the possibility of its presence 

 in the eight-cell stage. It attains its maximum size when the embryo 

 consists of twenty-four cells. He mentions the flattening of the blasto- 

 sphere which follows this maximum condition, but does not speak of an 

 obliteration of the cleavage cavity accompanying it, and considers that 

 the cavity is still present at the time the mesoderm cells sink below the 

 surface. He makes no suggestion of its recurrent nature at any period 

 of its existence. 



Brooks ('80, p. 80) mentions in Physa the " presence of a lens-shaped 

 segmentation cavity, which is enclosed peripherally by the union of the 



