The Early Development of the Pigeon's Egg. 19 



4. Between ten and twelve hours after fertilization, the sperm 

 nuclei disappear. This does not occur at any fixed stage of develop- 

 ment, but is soon before or soon after the thirty-two celled stage. Cer- 

 tain figures and photographs in this thesis show eggs with fewer than 

 thirty-two cells in which the sperm nuclei are lacking from some sides 

 of the blastoderm, and yet in an egg of thirty-two cells (shown in 

 Chart IV) they are still numerous, 



5. After the disappearance of the sperm nuclei, the marginal 

 cells become open peripherally and ventrally, and continuous with 

 the yolk. N^uclei from the marginal cells pass into the periblast 

 and the latter is, therefore, organized with nuclei derived from the 

 cleavage nucleus, and is exactly comparable with the periblast of 

 the bony fish, as described by Agassiz and Whitman ('84). 



III. POLYSPEEMY. 



Among those plants and animals whose fertilization has been 

 studied, the condition of monospermy is by far the more frequent, 

 so that the idea became prevalent tliat polyspermy was possible only 

 under pathological conditions. And many investigators have en- 

 deavored to solve the problem of why a spermatozoon, and usually 

 only one, enters an egg. It is quite possible that monospermy is 

 secured by different means in different species. But the most satis- 

 factory and most widely applicable explanation of this phenomenon 

 is that a physiological change takes place in the cytoplasm of the egg 

 immediately after the entrance of one spermatozoon, so that condi- 

 tions are not favorable for other spermatozoa. But polyspermy 

 as a normal condition has been described in a number of animals 

 and in at least one species of plants. Only one sperm nucleus, how- 

 ever, unites with the egg nucleus, and there is no conclusive evidence 

 that the supernumerary nuclei enter into the structure of the embryo 

 in any form, or take part in the formation of the germ layers. 



Polyspermy in Bryozoa. — Last year an interesting paper was 

 published by Dr. Kristine Bonnevie on "Physiologische Polyspermie 

 bei Bryozoen." Dr. Bonnevie ('OY) found compound spermatozoa, 

 or "Spermazeugmen." She argues that these compound structures 

 are not necessary to secure the power of locomotion for the sperms, 



