ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 19 



however, it lies almost exactly at that pole; in sections this appearance might 

 be due to obliquity of the plane of section to the egg axis, but in preparations of 

 entire eggs it can he seen that the spermatozoon does sometimes enter almost ex- 

 actly at the vegetal pole. It is unquestionably true that the point of entrance is 

 usually eccentric as Castle affirms, hut the degree of eccentricity just as certainly 

 varies in different cases. It might he supposed that this eccentricity always lay in 

 a single dehnite meridian, were it not for the fact that in cases of dispermy and 

 polyspermy the various points of entrance lie in different meridians {cf. figs. 12, 94). 

 I conclude therefore that the spermatozoon may enter at any point on the vegetal 

 hemisphere within about 30 of the pole. 



The fact that the spermatozoon always enters near the vegetal pole must be 

 due to some structural peculiarity ; the peripheral layer of protoplasm is a little 

 thicker at this pole than elsewhere at the time that the sperm enters, and this might 

 be held to be the cause of the sperm's entering at this pole, were it not for the fact 

 that the sperm enters at the vegetal pole in many other eggs, e.g. those of annelids 

 and mollusks, in which there is no periperal layer. It is probable that this very 

 general phenomenon is dependent upon some fundamental property, such as the 

 polarity of the egg or the direction of movement of the egg substance. 



2. Movements of Ooplasm. 

 With the entrance of the sperm the most astonishing series of changes takes 

 place in the egg. These changes are most striking in the living eggs of Cynthia, 

 where, owing to the yellow color of the peripheral protoplasm, the movements of 

 the egg substance can be directly observed ; but they may also be seen in the living 

 eggs of Ciona, and a detailed study of these changes may be made on fixed and 

 stained preparations. Almost immediately after the entrance of the spermato- 

 zoon the peripheral layer of protoplasm, which is nearly uniformly thick, and the 

 great area of nuclear plasm, in which the first maturation spindle lies (figs. 77, 78), 

 How around to the lower pole of the egg, leaving the first maturation spindle sur- 

 rounded by only a small amount of protoplasm. Thus within some ten minutes 

 after the entrance of the sperm the protoplasmic pole of the egg is transformed into 

 the yolk pole and vice versa. Castle does not figure nor describe this flowing of the 

 protoplasm from the animal to the vegetal pole, and it is probably owing to the fact 

 that he had not observed the early stages in which this occurs that he describes the 

 polar bodies as being formed at the yolk pole of the egg and the spermatozoon as 

 entering at the protoplasmic pole. Although he says that the presence of a sperma- 

 tozoon cannot be detected in the egg from which his figure 1 is drawn, I should sup- 

 pose from the fact that the first polar body is being extruded that the sperm must 

 already have entered {cf. my fig. 173). 



a. Localization of Yellow Protoplasm. 

 In Cynthia this downflow of protoplasm takes place so rapidly that it can 

 be seen in the living egg and with such force that the test cells, which lie between 



