24 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAX EGG. 



one near the egg nucleus; the latter afterward completely degenerates, according 

 to Castle, and takes no part whatever in the formation of the lirst cleavage spindle. 

 I have been unable to find tins archoplasmic mass in connection with the ega; 

 nucleus unless the remnants of the second polar spindle (fig. 87) maybe interpreted 

 as such. 



Any one \vli<> has studied the method of origin of the cleavage centrosomes in 

 the eggs 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 alter the entrance of the spermatozoon, and give rise directly 

 to the cleavage centrosomes; in the other, according to my observations, a 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 genu 

 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 v^^ are occasionally found into which two spermatozoa have- pene- 

 trated. The 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 I 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 dispermic eggs never divide, though 

 the nuclei may do so repeatedly, and of course they never develop normally. 



Dispermic 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, (lie two sperm nuclei 

 always enter the egg near the vegetal pole, and at lirst 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- 



