4M 



THE QUARTERLY REVIEW OF BIOLOGY 



heritance seems to indicate that the 

 "genes affect the cytoplasm, not immedi- 

 ately in a dynamic sense, but rather by 

 some material product that is set free from 

 the genes into the cytoplasm" (p. 655). 

 That there is a time element involved in 

 the action of the genes is also indicated 

 by other hybridization experiments, no- 

 tably those of Driesch on the sea urchin. 



Professor Morgan takes a decided stand 

 against assigning organ forming properties 

 to the various regions of the cytoplasm. 

 While he believes that at the beginning of 

 development the cytoplasm probably con- 

 tains certain factors which initiate 

 development, possibly determining the 

 cleavage type, he concludes from the 

 evidence furnished by the cleavage of egg 

 fragments that the segmentation pattern 

 is not foreshadowed or predelineated in 

 the protoplasm but that the form of 

 cleavage appears pari passu with the 

 development of the mitotic spindle. 



He is justly emphatic in his assertion 

 that the visible inclusions of the cytoplasm 

 have no organ-forming function since they 

 may, in many types of eggs, be displaced 

 without altering the course of develop- 

 ment. ' 'It does not follow, of course, that 

 there may not be other substances that do 

 have a determinative influence on develop- 

 ment" (p. 493). In commenting upon 

 the results of Conklin's experiments in 

 which the eggs of Styela were placed in 

 capillary tubes and centrifuged with the 

 consequent formation of abnormal em- 

 bryos with dislocated larval parts, he is 

 inclined to believe that alteration of the 

 cleavage by compression may be re- 

 sponsible. ' 'Whether the yellow pigment 

 can be separated by centrifuging from the 

 cytoplasm in which it lies, and may then 

 determine the local differentiation of a 

 region, must first be demonstrated in order 

 that crucial evidence of their differentiat- 

 ing function may be established" (p. 534). 



In discussing spiral cleavage, a type in 

 which the successive cleavages are more or 

 less rigidly predetermined so that "pre- 

 dictable regions of the egg pass into 

 definite cells," he states that "The 

 alternate changes from right to left and 

 then from left to right, etc., that are 

 characteristic of these divisions can hardly 

 be due to the presence of corresponding 

 regional substances already laid down in 



the egg The mosaic character of 



the cleavage with its accompanying local- 

 ization of differentiating factors is some- 

 thing given, but not preformed, in the 

 unsegmented egg" (p. 417). In other 

 words, the various cytoplasmic regions of 

 the egg are conceived of as predetermined 

 in many cases, but not prelocalized. 



Just what is implied by the term 

 "differentiating factors" is not entirely 

 clear. In certain cases, as he points out, 

 the character of the cytoplasm seems to 

 determine the character of the differentia- 

 tion of the regions of the body as, for 

 instance, in certain insect eggs, and as is 

 indicated by Boveri's experiment of cen- 

 trifuging the eggs of Ascaris, certain of 

 Spemann's constriction experiments on 

 Triton, etc. 



He points out, however, that "even in 

 such eggs where the cytoplasm plays an 

 important role in determining the regional 

 differentiation of the embryo, the charac- 

 ter of the nuclei is also determinative for 

 those characters that depend on their 

 constitution" (p. 42.4). 



It is interesting to note that symmetry 

 and asymmetry are discussed without 

 reference to Child's theory of physiologi- 

 cal gradients. Professor Morgan believes 

 that symmetry, in many cases, is impressed 

 upon the egg by outside influences and 

 would be loathe, we judge, to assume that 

 it is due to an inherent property of the 

 protoplasm in any case. We find ourselves 

 in hearty sympathy with his statement 



