VI 



DISPERMIC EGGS 



163 



have just discussed, for experimental embryology has established the fact 

 of the local differentiation of the cytoplasm of the egg (the so-called 

 " organ-forming " substances). This matter is discussed on pp. 188, etc. 



We have already found morphological evidence of the differentiation 

 of the chromatin in the genetic continuity of the chromosomes, the 

 constant size differences often visible between them, their composition 

 out of chromomeres of different but constant sizes, etc., and it is a 

 necessary corollary of the hypothesis of the dependence of Mendehan 

 phenomena upon the chromosomes (see below under (D)). Direct experi- 

 mental evidence of the functional differentiation of the chromosomes 

 has been obtained by Boveri (1907) from his remarkable experiments 

 on the development of polyspermic Echinoderm eggs. 



Among normally fertilized Echinoid eggs it occasionally happens 



Fig. 74. 



Triaster and tetraster mitotic figvires from a sea-urchin, Strongylocenirotus lividus. 

 (After Baltzer, Verh. Phys. Med. Gesells, Wmrzburg, iqo8.) 



that two spermatozoa enter the egg instead of one. By increasing the 

 concentration of sperm, the percentage of such dispermic (or poly- 

 spermic) eggs can be enormously increased. Thus in two parallel experi- 

 ments, eggs placed in water with only a few spermatozoa resulted in a 

 hundred normal monospermic and no di- or polyspermic fertilizations. 

 On the other hand, eggs placed in very concentrated sperm gave only 

 eleven monospermic and eighty-nine di- or polyspermic fertilizations. 



When an Echinoid egg is fertilized by two spermatozoa both sperm 

 nuclei (typically) fuse with the egg nucleus, and the centrosome intro- 

 duced by each spermatozoon divides as if it were the only one — hence 

 we get a zygote nucleus with 3^ (=54) chromosomes and four centro- 

 somes. A four-pole spindle figure is then produced {tetraster), and at the 

 first division the egg divides simultaneously into four blastomeres instead 

 of into two. 



In the most usual type of tetraster (and the only type which we will 



