246 The American Naturalist. [March, 



spindle, and therefore, the cleavage form of Jjera, to the constitutional 

 peculiarity of the ovum." « Holoblastic ova, the author believes, can 

 not be excluded from the action of external forces, but the presumption 

 is allowable, for several reasons, that even in these intrinsic forces are 

 important." The assumption must consequently be made " that intrinsic 

 forces reside in all ova, though they may be overshadowed by external 



It is important to examine the assumption which forms the founda- 

 tion of this argument. The author quotes E. B. Wilson's conclusion 

 that •« cleavage forms are not determined by mechanical conditions 

 alone," and assumes that by " mechanical conditions," Wilson means 

 conditions extrinsic to the ovum. This can hardly be so, for it is 

 necessary to include among the " mechanical conditions " influencing the 

 cleavage of an ovum like that of Jsera, the presence in the cytoplasm of 

 a great accumulation of food-yolk, (excessive in quantity when com- 

 pared with that in holoblastic or meroblastic ova). It is true that this 

 mass is within the ovum, and in so far " intrinsic ", but its action is 

 usually looked on as that of a foreign body, so to speak, which modi- 

 fies and obscures the primitive phenomona of cleavage and differentia- 

 tion as seen in holoblastic ova. Hence it is important to remember 

 that Dr. McMurrich, in maintaining that " the cleavage form of Jjera 

 is determined entirely by intrinsic conditions," must include the action 

 of the nutritive mass. This would seem to weaken materially the posi- 

 tion that extrinsic influences, (in the generally accepted sense as 

 extrinsic to active cytoplasm), are excluded from action on the spindles 

 of centrolecithal ova. The confusion seems to lie in the use of the 

 word intrinsic to include, in the case of the ovum of Jaera, both inher- 

 ent properties of the protoplasm, and secondary forces due to the pres- 

 ence of a body of nutritive material which is morphologically not a part 

 of the protoplasm. Dr. McMurrich's conclusion, that " instrinsic 

 forces reside in all ova", or preferably, as E. B. Wilson has just it, 

 " cleavage forms are not determined by mechanical conditions alone," 

 will probably be accepted as truth by most observers. However, I can 

 not see that he has shown that " in Jsera we have practically a demon- 

 stration of the correctness of this view " of a more convincing charac- 

 ter than is exhibited by holoblastic ova. 



"The cleavage form of Jsera, is said to be, determined entirely by 

 intrinsic conditions." A conclusion from which Dr. McMurrich sees 

 no escape, after a review of the changes of position in the yolk assumed 

 by the nuclei during segmentation. The Karyokinetic spindles then 

 are regarded as entirely beyond the influence of forces external to the 



