No. I.] POLYCHOERUS CAUDA TUS. 75 



they are formed before the egg is laid they are always retained 

 within it. Further, it was noted that, when first captured, 

 animals are more apt to lay than when long in captivity, even 

 under the most favorable conditions of fresh sea water, etc., 

 that I could devise. When, however, they are kept long 

 under rather unfavorable conditions, such as slightly stale or 

 too warm water, the dumb-bell-shaped structure disappears 

 by the drawing together of its extremities, and the nucleus 

 appears to return to its resting-stage. 



Sections show, however, that the true resting-stage is never 

 attained. The centrospheres still exist fairly distinctly, each 

 containing in its center a faint centrosome. The cytoplasmic 

 network, which, when the spindle is fully formed, is so startlingly 

 conspicuous, has disappeared, and the achromatic spindle fibers 

 have become very indistinct and shortened. The chromo- 

 somes have lost their cheveron shape, and in some cases seem 

 to have melted so as to form round bodies, and the outline of 

 the whole structure is but slightly oval, and occupies very 

 much less space than does the fully formed amphiaster. In 

 this stage, when examined in the living specimen, this retro- 

 grade amphiaster may readily be mistaken for an intact 

 nucleus. Sections show that it is but poorly defined in outline, 

 and apparently the greater portion of the material which 

 formed the amphiaster has changed its chemistry, so that it no 

 longer differentiates by stains as formerly. To what extent 

 this degeneration can take place, without the power of recovery 

 being destroyed, it is impossible to say, but the ova of worms 

 which have been kept for a long time under such abnormal 

 conditions sometimes fail to develop. 



This very unusual action of a spindle once fully formed sug- 

 gested immediately the "eigenthiimliche Art der Kernmeta- 

 morphose," noted first by Selenka (4) in the ova of Thysano- 

 zoon, and afterwards in other forms by Lang (5) and Wheeler 

 (6). Selenka describes in the uterine ova of Thysanozoon 

 diesingii a spindle which must strongly resemble in its action 

 that just described in PolycJiocnis cajtdatits. To quote him 

 directly, he says : " Nachdem das Ei seine definitive Grosse 

 erreicht hat, beginnt das Keimblaschen sich in typischer 



