52 Mary Blount. 



dotted lines in the diagram (Fig. 44a) indicate the region of the 

 periblast from which cells are added to the blastodisc. In the pigeon, 

 however, this region is of greater extent (Fig. 44b) and cells are 

 added to the blastodisc from the central periblast. Only the central 

 part of the latter does not contain nuclei, and does not proliferate 

 cells upward. I must explain Kolliker's description of the deepen- 

 ing of the blastodisc in this way. According to his description, 

 it is impossible to homologise the cleavage of the bird's egg with that 

 of other vertebrate eggs. Probably the deeper cells of the germinal 

 area of Duval's ('84) Figs. 7 and 8 are derived from the periblast, 

 and are to be regarded as holding the same relation to the cleavage 

 of the blastoderm as those added from the zone of junction to the 

 blastodisc in the teleost egg. 



Summary. 



A summary of the behavior of the supernumerary sperm nuclei 

 during the first twelve hours after fertilization was given on page 

 18 and need not be repeated here. 



1. Polyspermy. — Polyspermy has been reported in the Bryozoa, 

 Holothuroidea, Insects, Selachians, Amphibians, Reptiles, Birds, 

 and in one of the Gnetales. Polyspermy seems to take place because 

 of the lack of protection against it. The migration of the super- 

 numerary sperm nuclei in the pigeon (and perhaps in other forms) 

 is a response to the attractive influence of the unfertilized cytoplasm 

 in parts of the egg distant from the egg nucleus. The super- 

 numerary sperm nuclei perform no important function in the egg ; 

 their presence is a matter of chance ; and they are not essential to 

 normal development. The accessory cleavage is comparable to the 

 segmentation of fertilized enucleated egg fragments. 



2. Area of Primary Cleavage. — The blastodisc and periblast are 

 differentiated in the surface view in the maturation stage. The 

 early cleavage planes bear no constant relation to the axis of the 

 future embryo. Cleavage is not always asymmetrical. In the 

 surface view there are three concentric areas: (1) the central, (2) the 

 marginal area, and (3) the periblast. Cells are cut off centrally from 

 the marginal cells and are added to the central area until the former 



