594 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the chromosomes are usually abnormally and variously distributed in the 

 first cleavage of dispermic eggs, so that some cells obtain less, some more, 

 than the normal number. ^^ Boveri pointed out that the abnormal develop- 

 ment could not be due to cytoplasmic factors because 1/4 blastomeres of 

 normal eggs can give rise to normal larvae, and concluded that it con- 

 stituted proof of qualitative difference of different chromosomes. How- 

 ever, according to Schleip (1929, p. 473), frequency of normal develop- 

 ment of isolated primary three or four blastomeres of dispermic eggs is far 

 below expectation on the basis of chromosome distribution. Boveri ex- 

 plained this low frequency as due to injury in separation of blastomeres; 

 but Schleip suggests that, in addition to abnormal chromosome distribu- 

 tion and incidental injury, poor condition of eggs favors dispermy and 

 may be in part responsible for abnormal development. But whatever the 

 factors determining abnormal or differentially inhibited development in 

 dispermic eggs, the occurrence of normal development in some whole 

 eggs and isolated primary blastomeres shows that cytoplasmic pattern is 

 not necessarily altered. 



The modifications of cleavage pattern in dispermic eggs of the mollusk 

 Dentalium have been described by Schleip (1925). With simultaneous 

 cleavage into three or four cells, all may be in one plane or, in case of 

 four cells, may form a tetrahedron with three apical and one basal, or 

 one apical and three basal. Tetrahedral forms without definite relation to 

 the polar axis were not observed. The swimming forms which develop 

 from dispermic eggs of isolated blastomeres of these eggs are, according 

 to Schleip, too abnormal to permit exact analysis. 



Dispermy has been observed occasionally in Ascaris eggs and usually 

 results in simultaneous division into four cells. Fusion of two or even three 

 of these cells may occur, supposedly because chromosomes are absent be- 

 tween certain poles of the tetrapolar spindle. When the four cells persist, 

 three types of second cleavage are distinguishable, as follows: three cells 

 undergo diminution of chromatin, one does not; two cells undergo diminu- 

 tion, two do not; one undergoes diminution, three do not. The three pat- 

 terns are indicated in Figure 185, A-C, the cells which undergo diminu- 

 tion being uppermost. In these patterns three, two, or one of the four cells 

 correspond to AB, and one, two, or three, to P^ of normal cleavage, not 



^9 Boveri, 1902, 1904, 1905, 1907. Morgan had suggested earlier that the high frequency of 

 abnormal development in eggs dividing simultaneously into three or four cells, whether in con- 

 sequence of dispermy or other conditions, resulted from difference in number of chromosomes 

 in cells. 



