22 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



3. Development of Sperm Nucleus and Aster. 

 Immediately after it has entered the egg the sperm head is rod-shaped and is 



frequently coiled or twisted on itself (figs. 74, 79, 173). It decreases in length and 

 increases in width very quickly, and soon appears pear-shaped, the pointed end 

 being directed toward the sperm aster. At first densely staining throughout, it stains 

 less and less* densely as it swells in volume, until finally the chromatic and achro- 

 matic constituents are easily distinguishable (figs. 80-87). During this process there 

 are no evidences of chromosomal vesicles, the nucleus constituting a single vesicle. 

 In some cases there is a faint line between the head of the sperm and the egg 

 membrane which represents the middle piece and perhaps a portion of the tail (figs. 

 74. 79). Very soon after the spermatozoon has entered the egg a small aster, 

 with central clear area and minute rays, appears in the position of the middle 

 piece, between the sperm head and egg membrane (figs. 80, 173). The sperm aster 

 then grows with great rapidity, the rays extend throughout the greater part of the 

 clear protoplasm and even into the yolk and a minute body, the centrosome, becomes 

 visible at the centre of the rays, while the whole aster stains moi'e deeply that the 

 surrounding protoplasm (figs. 81-87). 



4. Path of the Spermatozoon within the Egg. 



The spermatozoon usually enters the egg in a radial direction and keeps right 

 on through the protoplasm at the lower pole until it reaches the deeper lying yolk 

 (figs. 74, 75, 80). This may be known as the penetration path (Roux). The sperm 

 nucleus and aster then rotate so that the aster is directed forward in all further 

 movements, as is true in so many other cases (figs. 80-83). The path described 

 after the rotation is the copulation path (Roux), and it always forms more or less 

 of an angle with the penetration path. While the penetration path may apparently 

 lie in any portion of the lower hemisphere within about 30 of the pole, the copula- 

 tion path seems to be definitely determined by the structure of the egg. The sperm 

 nucleus and aster move in this path from the neighborhood of the lower pole up to 

 the equator of the egg on the posterior side, all the time keeping near to the surface 

 of the egg (figs. 81-87). But this path is not always the shortest path to the 

 equator; sometimes it is the longest, as in figures 81 and 85, in which the sperm 

 having entered to the left of the lower pole moves across to the right side in the 

 figure and then up to the equator. The point near the equator to which the sperm 

 nucleus moves invariably marks the posterior pole of the egg and of the future em- 

 bryo, and the copulation path by which the sperm nucleus reaches this posterior pole 

 must lie along the posterior side of the egg ; but since the point of entrance of the 

 sperm and the penetration path may lie near to or far from the posterior side, it is 

 evident that they can have nothing to do in determining the position of the posterior 

 pole. And since the copulation path is not always the shortest path to the equator, 

 but may sometimes be the longest, it seems probable that the direction of the copu- 

 lation path is not the cause but the result of the antero-posterior differentiation of 

 the egg. A further consideration of this subject will be found in the general part 

 of this paper. 



