CTENOPHORA 



141 



corresponds, according to Metschnikoff (No. 16), to the so- 

 called upper or future sensory pole. Furthermore a histo- 

 logical difference between the smaller and larger blastomeres 

 of this stage is said to be noticeable, inasmuch as a larger 

 amount of ectoplasm is involved in the formation of the 

 smaller cleavage spheres. 



We have designated the furrows appearing up to this time 

 as meridional, because they had the same direction as the 

 chief axis. The next one to appear is, on the contrary, per- 

 pendicular to the chief axis (Fig. 65 if), and must therefore 

 be called an equatorial furrow. The formative yolk collects 

 in the upper part of the eight cleavage spheres, and is con- 

 stricted off in the form of small cells (Fig. 65 H), so that in 

 this way we obtain a stage composed of eight macromeres, 

 consisting almost exclusively of food-yolk, and eight micro- 

 meres (Fig. 65 I). Since in many instances the cells of the 

 embryo in this stage separate from one another, leaving a 

 space at the centre, the fundament becomes annular, a ring 

 of eight micromeres resting upon a larger one of eight macro- 

 meres. The cavity formed at the centre, whicli is open 

 above and below, as in the eight-cell and sixteen-cell stages 

 of Sycandra raphanus, we must designate as a cleavage 

 cavity (blastocoele). 



Subsequently a rapid multiplication of the micromeres 

 takes place, on the one hand by division of those already 



Fig. 66.— Three cleavage stages of a Ctenophore egg (diagrammatic), mi, micro- 

 meres; ma, macromeres (from Lang's Xe?iibucJi). 



present (Fig. 65 K) and on the other hand by the abstric- 

 tion of new micromeres from the underlying macromeres 

 (Fig. 66 B and G). In this way the annular cell-mass of 

 micromeres continues to spread out, and finally rests like a 



