300 



GENERAL EMBRYOLOGY 



is not altogether purely observational; there is some experi- 

 mental evidence as well, although largely indirect and possibly 

 none is positively conclusive. 



For example, Boveri has described the results of dispermy in 

 the sea-urchin egg. When two spermatozoa enter the ovum the 

 result is frequently the formation of three or four centrosomes 

 and asters connected with one another by spindles, upon which 

 the chromosome groups are usually drawn in abnormal com- 



FIG. 139. Larva of Sphcer echinus, derived from a dispermic egg, showing 

 differences in nuclear size, distribution of pigment, etc. The dashed line marks 

 the separation of the two portions of the larva. After Boveri (reconstructed 

 from two figures), n, small nuclei; N, large nuclei; p, pigment. 



binations, so that when such an ovum cleaves it separates 

 into three or four cells containing nuclei whose composition is 

 therefore abnormal. In many such cases Boveri finds that 

 each of the three or four cells forms a group of cell descendants 

 which can be identified by the presence or absence of certain 

 characters or by unusual combinations of characters (Fig. 139), 

 so that the entire embryo may be said to consist of three or 

 four regions, each with certain distinctive characteristics. 

 Furthermore, in some instances, the cells of the various fractions 



