No. 3.] THALASSEMA AND ZIRPHAEA. 593 



cate that the difference between these extreme cases may be 

 primarily only that of position of the asters. 



Still another fact has to be considered as influencing the 

 length of the spindle. By average of actual micrometer meas- 

 urements (full account being taken of the above-mentioned 

 variation in the distance between the asters) it was found 

 that the completed spindle is approximately one-fourth sliorter 

 than the distance between the centrosomes previous to meeting 

 of the rays — i.e., the asters approacli at completion of the spindle. 

 This often causes the asters to become more or less imbedded 

 in the nuclear skein. A shortening of both maturation-spin- 

 dles occurs in Ascaria (Boveri, '87), Arteniia (Weismann und 

 Ischikawa ('88), Ophryotrocha (Korschelt, /. c), and Branchipiis 

 (Brauer, '82), and is explained by the first and last named authors 

 as a result of contraction of the spindle-fibers. In Thalasscma, 

 however, the shortening takes place at a much earlier stage, 

 probably while the chromosomes are still severally connected 

 with but one of the asters. It more nearly resembles Mac- 

 Farland's ('97) account of the approach of the cleavage-asters 

 to form the cleavage amphiaster in Pleuropliyllidia. Erlanger 

 ('98) has recently described a shortening of the cleavage-spindle 

 just previous to metaphase in the sea-urchins. 



From its completion (PI. XXXI, Fig. 12) until the polar 

 body is separated, the first maturation-spindle maintains very 

 nearly a constant length, measuring about \ the egg-diameter. 

 A few measurements made during the expulsion of the polar 

 body show that also during this process the spindle retains a 

 fairly constant length, and hence must rise bodily with the 

 elevation of the protoplasm that gives rise to the polar body. 

 This has been also observed in Physa (Kostanecki and Wier- 

 zejski, '96). At this stage the egg, as a whole, elongates in 

 the direction of the spindle-axis only ^^ of the mean diameter. 



The development of the asters, the progressive increase in the 

 number, length, and strength of the rays which at first throw 

 the nuclear membrane into folds and then rupture it and enter 

 the nucleus (PI. XXXI, Figs. 7-12), have all been described 

 in a previous paper (Griffin, '96). During the early stages of 

 the asters, while they are commencing to break through the 



