42 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



.it 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. 23), thus 

 forming a deep yellow semicircle in each blastomere. The intrusion of yolk may 

 still be seen entering this semicircle 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 ooplasm ; not only does the cres- 

 cent take pai't 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- 

 27). I have been unable to analyze all of these movements; one thing, however, 

 seems very probable, viz., 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 eggs 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) can 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 nuclei, 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 egg 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.^7 seq.). 

 Castle has observed this telokinetic movement in Ciona, and describes it in the fol- 

 lowing words (1896, p. 233): "The first cleavage spindle arises, as has been stated, not 

 far from the center of the egg. As its first cleavage is nearing completion, however, 



