24 ORGANIZATION AND CELL-LINEAGE ОЕ ASCIDIAN EGG. 
one near the ege nucleus; the latter afterward completely degenerates, according 
to Castle, and takes no part whatever in the formation of the first cleavage spindle. 
I have been unable to find this archoplasmic mass in connection with the egg 
nucleus unless the remnants of the second. polar spindle (fig. 57) may be interpreted 
as such. 
Any one who has studied the method of origin of the cleavage centrosomes in 
the eges of ascidians and of mollusks cannot fail to be impressed with the profound 
differences between the two. In the one we have no centrosome or aster in connec- 
tion with the egg nucleus at any stage, while the sperm centrosome and aster are 
visible at all stages after the entrance of the spermatozoon, and give rise directly 
to the cleavage centrosomes; in the other, according to my observations, а centro- 
some and aster are found in connection with each of the germ nuclei, and coinci- 
dently with the union of these nuclei the asters or spheres also unite, while out of 
this fused sphere material a single centrosome arises in connection with each germ 
nucleus. It is recommended to those who maintain that in these details of fertili- 
zation all animals must conform to a single type that they study the fertilization of . 
a gasteropod as compared with that of an ascidian. 
7. Dispermy. 
Although it is a relatively rare thing for more than one spermatozoon to enter 
an egg, still eggs are occasionally found into which two spermatozoa have репе- 
trated. Тһе entrance of more than two spermatozoa, if it occurs at all, must be a 
very rare phenomenon. In stained preparations and in serial sections I have never 
seen an undoubted case of it; unsegmented eggs are sometimes found in which 
there are a number of nuclei, but in all cases it is possible that these may have 
arisen from the division of two sperm nuclei. In living eggs 1 have sometimes 
observed several yellow spots on the lower hemisphere. Such an egg is shown in 
figure 12; there are here four yellow spots, each about equidistant from the vegetal 
pole, and presumably there is a nucleus in each of these, though nuclei were actually 
observed in only two of them. It is possible that these may have arisen by divi- 
sion from two original nuclei, and that this is therefore a case of dispermy and not 
of polyspermy. It is an interesting fact that dispermie eggs never divide, though 
the nuclei may do so repeatedly, and of course they never develop normally. 
Dispermie eggs have been repeatedly observed both in living and in fixed 
material, in entire preparations and in serial sections. Such eggs afford a valuable 
means of testing the question as to whether the point of entrance of the sperm is 
predetermined, and more important still, as to whether the posterior pole of the 
egg and the plane of the first cleavage is pre-existent in the egg or is established 
by the entering spermatozoon, So far as I have observed, the two sperm nuclei 
always enter the egg near the vegetal pole, and at first they lie in a common proto- 
plasmic field. As they move toward the equator, however, they frequently sepa- 
rate, and when they have reached the equator and have each given rise to a spin- 
dle they are often found on opposite sides of the egg with the surrounding proto- 
