. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 60 L. V. HEILBRUNN AND W. L. WILSON might be minor fluctuations in viscosity, but the weight of evidence is against any change whatsoever. This may seem strange, for in the egg of the clam Cumingia one of us had noted marked changes in viscosity during the maturation divisions (Heilbrunn, 1921). The difference between the egg of Chaetopterus and that of Cumingia is, however, easy to understand. In the relatively small egg of Cumingia, the maturation spindles occupy a rather large fraction of the egg volume. Thus Morris (1917
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 60 L. V. HEILBRUNN AND W. L. WILSON might be minor fluctuations in viscosity, but the weight of evidence is against any change whatsoever. This may seem strange, for in the egg of the clam Cumingia one of us had noted marked changes in viscosity during the maturation divisions (Heilbrunn, 1921). The difference between the egg of Chaetopterus and that of Cumingia is, however, easy to understand. In the relatively small egg of Cumingia, the maturation spindles occupy a rather large fraction of the egg volume. Thus Morris (1917) states that the first polar spindle "is large, and lies near the center of the egg. It might, in fact, be mistaken for a cleavage spindle in the metaphase, if it were not for the form of the ; Similarly, Jordan (1910) shows the first maturation spindle of Cumingia as a large structure extending through most of the egg; the distance between the centers of the centrospheres of this spindle is approximately half the diameter of the egg. On the other hand, the 15 >- 12 S9 to >' 6. I I I I I I 5 10 15 20 25 30 35 40 45 50 55 MINUTES AFTER FERTILIZATION FIGURE 1. Protoplasmic viscosity changes in the egg of Chaetopterus during the time between fertilization and first cleavage. maturation spindles of the Chaetopterus egg are relatively small. Thus, Lillie (1906) shows the fully formed maturation spindle of the Chaetopterus egg as small. In his Figures 3 and 4, the. distance between the centers of its two centrospheres is only one-sixth of the diameter of the egg (Compare also Mead, 1898). Lillie (1906) showed that the cortex of the Chaetopterus egg was relatively viscous, con- taining granules which did not move when the egg was subjected to reasonably strong centrifugal force. We conceive, therefore, of the maturation spindles of the Chaetopterus egg as being relatively small bodies only several times as long as the cortical layer is thick and extending
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