164 CYTOLOGY CHAP. 



here consider) the two cleavage planes separating these first four blasto- 

 meres are identical in position with the two planes formed by the first 

 two cleavages of the normal monospermic egg. Hence the four simul- 

 taneously formed blastomeres of the tetraster are identical as regards 

 cytoplasmic contents with the first four blastomeres of the monospermic 



egg- 



By shaking the eggs immediately after fertilization it may happen 

 that the sperm centrosome fails to divide, and in the case of dispermic 

 eggs it often chances to be brought about that one centrosome divides 

 and the other does not. In this case the ^n zygote nucleus is provided 

 with three centrosomes and a triaster spindle results, the egg dividing 

 into three blastomeres at the first cleavage. 



Now, it is obvious that in the tetraster the 3^ chromosomes or rather 

 the products of their division that is to say, 6n chromosomes, have to 

 be distributed amongst four nuclei ; and as the arrangement of the 

 chromosomes on the various parts of the four-pole spindle seems to be 

 by chance, the number and combination of chromosomes received by 

 each nucleus are various. As an illustration, one of the very many 

 possible arrangements in the spindle, and the resulting number of chromo- 

 somes in the daughter nuclei, is given in Fig. 75. 



Even when a blast omere receives more than n ( = 18) chromosomes 

 there is still a strong chance that it may be lacking in one or more 

 members of the series. Among the four nuclei, the daughters of three 

 members of each series are to be distributed that is to say, there are 

 two (sA+ 36+30+ . . . 3R) to be distributed among the four nuclei. 

 Hence in the vast majority of cases one or more of the blastomeres will 

 lack a representative of one or more members of the complete series of 

 chromosomes. A pictorial example of one out of the many possibilities, 

 showing only four of the eighteen chromosomes, is given in Fig. 75, D, E. 



Once the first four blastomere nuclei have been constituted, of course 

 the number and series of chromosomes contained in them will be per- 

 petuated in all the nuclei descended from them. 



Now, it has been established by many workers, including Boveri 

 himself, that if the first four blastomeres of the normal Echinoid egg are 

 separated (which can most conveniently be effected by placing the 

 egg in sea water from which the calcium salts are lacking as discovered 

 by Herbst) each will develop into a dwarf pluteus of one quarter^ the 

 normal size. The plutei generally indeed have minor defects, and the 

 four plutei derived from a single egg may exhibit differences from each 

 other, but these are comparatively slight. Hence the great abnormalities 

 among the embryos developing from isolated blastomeres of tetraster 

 eggs, which will be described immediately, cannot be ascribed to their 

 cytoplasm, since this does not differ from that in the first four blastomeres 



