ox DEVELOP^IEXT ETC. OF PHORONIS. 515 



to ii;r:i(lnnlly ohsr-nvo tlie orderly ;irr:iii<;-oinont of tlie ec'll^<. At tlie 

 ir)-cc'll stiiiTO tlio reo-ular arnniii'emont is still, thono-li less distinctly, 

 iiiaiiitained, while at the 32-pell stage it is quite distiirlted (fig. 4), 

 From this period on, the polar globnles can no longer be detected. 



In the earlier stages of segmentation, the blastomeres are 

 found in close contact witli one another, leaving no noticeable 

 space or segmentation-cavity between them. After they have 

 increased to about 32 in number, the blastocœle and its opening 

 to the exterior {ùii:. 4, /V.r.) become recognizable. The emliryo 

 at the morula stage is somewhat oblong in shajx- and has 

 a (juito spacious blastocœle, and the blastoccelic pore (/>/.c.) 

 is distinct on the ventral side (fig. 5). However, this pore dis- 

 appears at an advanced morula stage, and a])parently the vitelline 

 membrane also, nearly simultaneously with it. At any rate both 

 have altogether passed out of sight at the next stage, that of the 

 blastula. In fig. 23, which represents a median section of a young 

 morula (the outline of which has undergone mutual compression 

 by the crowding together of emlu'vos), the pore (Ij/.c.) is cut through 

 and appears as a slit-like passage between two of the bounding 

 blastomeres. 



In the blastula (fig. 2-3) the wall consists of cylindrical cells 

 and encloses a tolerably wide blastoca^lic ca\ity, which is now at its 

 greatest development. In this stage, the bilateral symmetry of 

 the future larva is already established. It has an oblong ]>lano- 

 convex form, the fiatteued face of which corresponds to the future 

 ventral f;ice ; and its ends, one somewhat bi'oader than the other, 

 indicate respectively the future anterior, and posterior, ends. 

 The cells of the wall are all cylindrical in form, as shown in fig. 

 2Ö, those of the ventral side l)eing slightly larger than those on 



