EMBRYOLOGV OF CLEPSINE. 253 



vesicle into the archiamphiaster, which event marks the beginning 

 of the period of proliferation. It is therefore initiated and, in 

 all probability, sustained throughout its various phases by 

 chemico-physical changes originating in the nucleus itself. 



The most remarkable example of this polarity hitherto de- 

 scribed, is furnished by the egg of Clepsine. The period of 

 unipolar activity is introduced and maintained throughout at 

 the expense of nuclear changes ; and the period of bipolar acti- 

 vity, beginning with the formation of the pronuclei, reaches its 

 climax soon after their union, and then gradually subsides. The 

 scene of action is now transferred from the poles of the egg to 

 the poles of the cleavage-amphiaster. Here as before, the energy 

 displayed is at first weak, but rises gradually and culminates in 

 sundering the hemispheres of the egg. A. repetition of the same 

 process divides the hemispheres into four quadrants. It is inter- 

 esting to note that these phenomena are displayed successively 

 at the poles of different axes, each of which cuts the preceding 

 at right angles. 



(1.) Polar Rings. — It is very remarkable that nothing has 

 yet been observed by embryologists which can with any degree 

 of certainty be compared with the polar rings before described. 

 It is hardly possible to beheve that these rings and rays, in 

 which at one epoch the vital energies of the egg seem concen- 

 trated, have no parallel in the eggs of other Metozoa. They 

 would be easily overlooked in transparent eggs and recognized 

 with difficulty in opaque eggs, if faintly expressed. The fact 

 that they form so conspicuous a feature of egg-life in Clepsine, 

 makes it extremely probable that they occur ni others, at least 

 in closely allied animals, such as Nephehs, Hirudo, &c. Kow- 

 alesky (ttV—j) has described a clear elliptical spot, found on 

 one pole of the egg of Euaxes, which 1 am inclined to believe 

 represents the remains of a ring. This body was found on 

 fresh laid eggs, and was supposed to represent the germinal 

 vesicle, which was no longer recognizable in the interior of the 

 egg. This spot marks that side of the egg on which the clea- 

 vage-dej)ression first appears, and is always found on the larger 

 part at the close of the cleavage — precisely as in Clepsine. Its 

 disappearance after the fir^t division is thus described .• — " Das 

 Verschwinden konnte leicht beobachtet werden ; die Rander des 

 h- lien Fleckes wurclen immer mehr und mehr unregelmiissig, so 

 zu sagen zerfressen, da in das belle sie zusammen-setzende Pro- 

 toplasina feine Dotterkornchen eintraten ; der ganze Inhalt des 

 Fleckes triibte sich weiter und verschwand endlich vollstandig." 

 A small globule was found resting on this body in eggs treated 

 with chromic acid, and was interpreted as an artilicial product, as 

 it was not seen on fresh eggs. The entire description corre- 



