OF ARTS AND SCIENCES. 45 



the ends ; and at the end of eleven minutes, reckoning from the mo- 

 ment of contact, the coalescence is complete, and we have a perfect 

 sphere .02 mm. in diameter. Within one minute this sphere under- 

 goes a transformation, which ends iu the formation of the first cleav- 

 age-amphiaster. This amphiaster has a horizontal position, at right 

 angles to the axis of the ovum. From the moment when this trans- 

 formation begins to the moment when the first cleavage-groove ap- 

 pears, is only eight minutes. Thus the conjugation of the pronuclei 

 and the formation of the first cleavage-amphiaster require twenty 

 minutes. We have found very little variation from these figures. 



From the above measurements it is evident that the pronuclei in- 

 crease somewhat in volume during the few minutes (10-12) con- 

 sumed in the conjugation. A similar fact was observed by Btitschli 

 in the ovum of Nephelis (No. 44, p. 6), and more recently by Mark 

 in the ovum of Liniax (No. 28, p. 220). 



In the mounted preparation just referred to, I can discover no 

 distinct astral lines around the body which we regard as the male 

 pronucleus.* 



It is an interesting fact, that the first cleavage-spindle is parallel 

 with the plane of junction of the pronuclei, precisely as in the ova of 

 many invertebrata. If this spindle were vertical, as supposed by 

 Hoffmann, we should expect to see a rotation of the pronuclei, like 

 that described by Auerbach iu the ovum of Rhabditis. No such event 

 occurs in the ova we are describing. 



The Velocity of Cleavage. — From the appearance of the first cleav- 

 age-groove to the moment of hatching is but little more than fifty 

 hours in any of the species we have followed. This period was only 

 forty hours in Ps. ohlongus. 



We have traced the genesis of the amphiastral figures to the ninth 

 generation in Ctenolabrus. The time that elapses between the first 

 cleavage-amphiaster and the ninth generation of amphiasters was 

 ascertained in one case to be two hours and sixteen minutes. The 

 time between successive generations of amphiasters diminishes as the 

 cleavage advances. Between the first and second generation twenty 

 minutes passed ; between the second and third, also twenty minutes ; 

 between the third and fourth, eighteen minutes ; and from this point 

 onward the time diminishes gradually, until, between the eighth and 

 ninth generations, only fifteen minutes intervene. 



* Mark (1. c, p. 221) finds that the male pronucleus (Limax) is accompanied 

 by an astral figure only in abnormal cases. 



