DIFFERENTIATION, HEREDITY, SEX 301 



may be characterized by unusually large or small nuclei, indicat- 

 ing the presence of larger or smaller amounts of chromatin 

 (numbers of chromosomes) than usual; the microscopic examin- 

 ation of these multipolar spindles shows that the chromosomes 

 may be distributed with great irregularity in the first division. 



More striking are the results following the separation of the 

 blast omeres of such dispermic eggs. The isolated cells of a 

 four-cell stage resulting from normal fertilization, develop 

 normally, producing four similar but small normal larvae 

 (Fig. 133). But the isolated cells of one of these three-cell 

 stages develop dissimilarly, each with certain defects; and just 

 as any possible combination of chromosomes may have occurred 

 in each of the three original cells, so all possible combinations 

 of characters are found in the larvae developing from such cells 

 when isolated. Boveri believes that this warrants the conclu- 

 sion that, while the presence or absence of certain chromosomes 

 may not result in the presence or absence of specific traits, yet 

 a certain combination of chromosomes is essential for normal 

 development, a fact which would mean only the physiological 

 specificity of the individual chromosomes. 



Perhaps the most striking experimental results are those 

 obtained by fertilizing the eggs of one species, with the sperm 

 of another species, genus, or even phylum. In the first place, 

 Boveri in 1889 reported that non-nucleated egg fragments of 

 Sphoerechinus (one of the sea-urchins), fertilized with the sperm 

 of Echinus, developed into larvae exhibiting only paternal 

 characters. This appeared to afford strong evidence that the 

 characteristics of the nucleus rather than of the cytoplasm 

 determine the course of development. Later attempts (See- 

 liger, Morgan, Boveri) to confirm these facts led to incon- 

 clusive results. Indeed exactly opposed results were obtained 

 by several investigators (Driesch, Loeb, Godlewski, Hage- 

 doorn). Eggs of one species of Echinoderm fertilized with the 

 sperm of another species, genus or class, of Echinoderm, or 

 even with Molluscan sperm, resulted in the development of 

 larvae possessing wholly or largely the maternal characters. 

 These results indicated just as strongly that the nuclear com- 



