24 Mary Blount. 



into the formation of the germ layers. But they disappear in such 

 an early stage of cleavage, that their participation in the structure 

 of the embryo is out of the question. 



Cause of Migration of Supernumerary Sperm Nuclei in the Pigeon, 

 The conditions in the pigeon's egg are not well explained by Riickert's 

 theory of expulsion. Harper ('04) suggests that the "sperm nuclei 

 migrate so early to the periphery of the germinal disc, that it is 

 difficult to believe that they do this under the influence of the cleavage 

 nuclei." I have shown in Chart I and Eigs. 2 and 3 that before- 

 the union of the pronuclei the supernumerary sperm nuclei have 

 migrated into the periblast. In speaking further of the early migra- 

 tion of the sperm nuclei,- Harper says, "This seems to point to the 

 independent activity of the sperm nuclei rather than to any mechan- 

 ical driving of them from the inner region. ^Vhat chemotactic in- 

 fluences there may be present, we, of course, have no means of know- 

 ing" (p. 3T8). In another place (p. 372), Harper says: "As an 

 active cause for the migration of the sperm nuclei it might be as- 

 sumed- that the activity is but the continued expression of the labile 

 nature of the protoplasm, which gives the sperm its motile character 

 during the period of its independent existence." I am willing to 

 accept for the pigeon Riickert's theory for the cause of polyspermy 

 in the selachian, namely, the want of protection against it. Harper 

 speaks of the "thinness of the egg membrane when it leaves the tough 

 ovarian capsule." It is commonly accepted among biologists that 

 the egg exerts a chemotactic influence on spermatozoa. Monospermy 

 is secured in many eggs by the cessation of the attractive influence the 

 moment that a spermatozoon enters the egg, i. e., the cytoplasm that 

 is fertilized no longer attracts spermatozoa. Wilson ('03, p. 418) 

 found that spermatozoa enter enucleated fragments of the unfertilized 

 nemertine egg, but they do not enter enucleated fragments obtained 

 after fertilization even in the absence of an egg membrane. 



I wish to use this theory of the attractive influence of the egg 

 not only to explain the entrance of the spermatozoa, but their migra- 

 tion into the periblast. A varying number of spermatozoa (Harper 

 found from 12 to 25 in fertilization stages) enter the pigeon's egg 

 in the vicinity of the egg nucleus. The cytoplasm, then, in this 



