140 



The Nucleus and Cytoplasm in Development 



sperm asters divide to form a mitotic figure 

 with four poles connected by spindles among 

 which the chromosomes are distributed at 

 random (Fig. 34b). During anaphase, the 

 four poles thus receive different numbers of 

 chromosomes which are incorporated into 

 four nuclei (Figs. 34c and d). Since these 

 nuclei divide normally in subsequent mi- 



tity of chromosome material that causes the 

 standstill of development, but the presence 

 of abnormal combinations of individual chro- 

 mosomes in one or more quarters of the 

 blastula or pluteus. Normal development be- 

 yond the gastrula stage requires the pres- 

 ence of nuclei containing at least one com- 

 plete set of chromosomes. 



Fig. 35. Diagram of development of polyspermic eggs of the newt, following removal of egg nucleus; 

 frequent disengagement of nuclear and centrosomal cycles producing abnormal mitoses, a. Five sperm 

 nuclei present, two of which are about to fuse, b, First mitotic cycle; tetrapolar mitosis at upper right, two 

 tripolar mitoses at left, monaster at lower right, c. Second mitotic cycle; several cells again contain abnormal 

 mitotic figures, d. Section through final stage of development reached; blastula with wide variety of chromo- 

 some numbers in different areas. 



toses, the abnormal chromosome sets are re- 

 produced and handed on to all the cells 

 descended from each of the first four blasto- 

 meres. A blastula results which consists of 

 four areas differing in chromosome num- 

 ber as is shown by differences in the size of 

 the nuclei; the individual cells are too small 

 to allow actual chromosome counts at this 

 stage. Some of these eggs never develop be- 

 yond a "stereo-blastula" in which one or 

 more quarters disintegrate, causing the death 

 of the whole embryo (Fig. 34e). Others may 

 form abnormal plutei in which again a 

 regional breakdown of the cells takes place. 

 Boveri ('02, '07), in a masterly analysis of 

 the development of dispermic eggs, showed 

 that it is not a deficiency in the mere quan- 



In polyspermic eggs and egg fragments of 

 newts from which the egg nucleus has been 

 removed, multipolar and other abnormal mi- 

 toses occur frequently. Because of the large 

 size of the eggs, several mitotic figures may 

 be present in the same egg and cause the 

 formation of multiple cleavage furrows (Figs. 

 35a and b). The cytological picture is com- 

 plicated still further by the fact that multi- 

 polar or monocentric mitoses may again occur 

 in individual blastomeres during the second 

 or a subsequent mitotic cycle (Fig. 35c). 

 Counts of chromosomes in the blastula re- 

 veal a highly complex pattern of abnormal, 

 mostly aneuploid, chromosome nvimbers (Fig. 

 35c?). Since, in Triturus, most of the 12 

 chromosomes of the haploid set may be rec- 



