42 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
at intervals of about two or three minutes. The crescent is first constricted in the 
middle (figs. 20 and 21); then the gray yolk penetrates into the lower part of each 
half of the crescent and approaches near to the surface, being covered only by a 
thin layer of the yellow protoplasm (fig. 22). Above and below this intrusion of 
yolk the crescent remains deep yellow in color; in the region of the intrusion the 
color is gray with a superficial covering of yellow pigment granules. The lower 
(dorsal) portion of the crescent thus cut off from the remainder is small as compared 
with the upper part, and is median in position. Not more than two minutes after- 
ward this lower part unites with the upper along the median line (fig. 25), thus 
forming a deep yellow semicircle in each blastomere. Тһе intrusion of yolk may 
still be seen entering this semicirele through its open half, which is dorsal and lat- 
eral in position. Then these two semicircles come into contact with each other 
along the first cleavage plane, the free ends enlarge into rounded knobs, and the 
intrusion of yolk is less large (fig. 24). Finally, the intrusion becomes still smaller, 
the open ends of each semicircle join, and the crescent is reestablished (figs. 25 and 
26). Observation of the living egg during this period of division gives the impres- 
sion of remarkable cytokenetic activity in all the oóplasm ; not only does the cres- 
cent take part in this activity but the yolk and the clear protoplasm undergo 
marked movements, in the course of which the clear protoplasm is divided into two 
areas which are entirely separated from each other by a partition of yolk (figs. 25— 
21). I have been unable to analyze all of these movements; one thing, however, 
seems very probable, vzz., that they are in the main of a vortical nature and that 
they are comparable with the movements in the constriction of the cell body which 
I have observed in gastropods (Conklin, 1902). I have not thoroughly studied 
these movements by the aid of serial sections; since they take place with such 
rapidity, this would be possible only by sectioning and studying a very large number 
of eges during the period of the first cleavage. In figure 99, which is an equatorial 
section of an egg of the stage shown in figure 22 or 23, the substance of the crescent 
(Cr) ean be recognized on the posterior side of the egg; it does not, however, show 
any of the thickenings or thinnings indicated in the surface views mentioned. 
Beneath the superficial layer of yellow protoplasm in this figure is an area of clear 
alveolar protoplasm, while still deeper is the radiating protoplasm which constitutes 
the astral systems. 
In the telophase of the first cleavage the centrosomes, daughter ише, and the 
surrounding areas of clear protoplasm rotate toward the animal pole (fig. 100) in a 
manner similar to that which occurs in the blastomeres of gasteropods (Conklin, 1902). 
Through the agency of these telokinetic movements of the first cleavage the nuclei, 
centrosomes and clear protoplasm are carried above the equator of the ege toward 
the animal pole. The pole at which the polar bodies lie thus becomes more richly 
» protoplasmic than the opposite pole and ever after continues to be so (fig. 102, et seg.). 
Castle has observed this telokinetic movement in Czoza, and describes it in the fol- 
lowing words (1896, p. 233): “Тһе first cleavage spindle arises, as has been stated, not 
far from the center of the egg. Ав its first cleavage is nearing completion, however, 
дық eS hiii aia Aie е RE а аад ымыы. 
E" 14 
