132 A. OKA. 



3. Development of the Polyzoöid 

 in the Statoblast. * 



As the mature statoljlast floats on the surface of water, the belt- 

 plate of the annuhis splits horizontally, so that tlie shell may now l)e 

 said as being composed of two valves. These however remain tight- 

 ly apposed during winter. On the arrival of warm temperature, they 

 separate from each other, l)ut holding the whitish contents between. 

 The two valves have then very much the a[)pearance of a pair of 

 cymbals. The separation takes place at a stage when no change is 

 yet percejitible in the contents ; hence I am inclined to ascribe its 

 cause to some external influence rather than to internal pressure. 



The contents of the statoblast, i, e., the granular mass with its 

 enveloping epithelium, form a spheroidal mass. All along the outer 

 marö-in or the equator of the spheroid, where the separation of the 

 sliell-valves has brought it in direct contact with water, the enveloj)- 

 ing epithelium becomes thicker (figs. 48, Out. lay. and 48 A, PI. XX.), 

 owing to increase in height of cells, accompanied by great increase 

 in size of tlie nuclei, which are now as large as those of grown-up 

 polypides. The process of thickening thus begun at the equator 

 proceeds gradually toward the two poles of the spheroidal mass, so 

 that the membrane (hickens latest at these places. 



Meanwhile, the cells at two opposite areas on the equator 

 become especially taller, so that the enveloping membrane acquires 

 a marked thickness at these places. The areas in question are oblong 



* Aftoi- finishing the manuscript of this paper, I received No. 324 (1889) of the Zoolo- 

 gischer Anzeiger containing Braem's preliminary report entitled " Die Entwick]. d. Urj-ozoen- 

 colonie im keimenden Statobl." His statements differ in many fundamental points from mine. 

 There is sufKcient ground to assume that very considerable variation of development obtains 

 among different speci« s of Poljzoa. 



