210 THE AMERICAN MONTHLY [November, 



Isotropy. — Pfliiger, In 18S3, tried to show that the direction of the cleavage 

 planes in the amphibian egg was determined by gravity, and that the germ 

 alwavs gave rise to the same form because it developed under the same con- 

 ditions. These claims must have been correct had the egg been not self- 

 directive, but they have not been allowed, and b}- experiment the cleavage 

 planes were proved to be while not independent of gravity, at least not 

 primarily caused by its influence. 



When this view, that the entire egg was merely passive, had been abandoned, 

 writers such as Born (18S4), Hertwig (1885), Wiesmann (1S85), and Kolli- 

 ker (1S85) suggested that perhaps the nucleus was the directive part of the 

 &^'g^ but that the cytoplasm was purely passive. In this form the doctrine 

 of Isotropy may be nearer the truth, but it does not express the entire truth. 

 Its adherents cite in its favor the conspicuous part played by the nuclear bodies 

 in the fecundation and segmentation of the egg ; also, the incapacity for regen- 

 eration on the part of enucleate protozoa. Great light has been thrown upon 

 the opposite view, that of the self-dii-ective activity of the cytoplasm, by Van 

 Beneden in his researches upon the development of Ascaris. The truth ap- 

 pears to lie between the two extremes in ascribing to both the nucleus and the 

 cytoplasm a self-activity and a share in determining later events. One fact 

 vvhich tends to prove the participation of the cytoplasm is the presence in 

 Clepsine of polar rings in the cytoplasm at the same time that nuclear phe- 

 nomena are displayed. That the former is not caused by the latter is shown 

 by the fact that they begin before the nuclear movements, and because they 

 are unknown in other eggs, and for the second reason, that they are not due 

 to pronuclear influences. 



Cytokinetic phenomena * — These are numerous, and many of them are 

 plainly associated with nuclear motions or karyokinesis. Whether any are 

 independent or originate otherwise than in karyokinetic actions is not certain. 

 The facts in the case are not sufficiently known to furnish a complete solution 

 to the problem, but a few examples will be next examined. 



The exit of polar globules is often attended by a considerable flattening of 

 the eg^', followed in some cases by a remarkable constriction which traverses 

 the egg from the equatorial zone to the animal pole, finishing up with a nip- 

 ple-like prominence from which the first polar globule is expelled. The 

 second polar globule is sometimes accompanied by a similar but weaker cyto- 

 plasmic movement. This has been seen in Clepsine, Petromyzon, and certain 

 Teleosts. Thus, here we find the expulsion of the polar globules a part of 

 maturation to involve the cooperation of two factors — one karyokinetic and 

 one cytokinetic. 



Cytoplasmic movements are also observed in pelagic fish ova. In these the 

 cvtoplasm at first forms a zone of even thickness around the whole ovum. 

 Upon the entrance of the spermatozoon this layer of protoplasm concentrates 

 itself at one pole of the egg, and from it the germinal disk is formed with its 

 centrally-placed male pronucleus. 



The artificial division of infusoria has been resorted to in the attempt to 

 show the relative importance of the nucleus. Nussbaum (1S86) was first to 

 show that enucleate pieces of infusoria were incapable of regenerating lost 

 parts, while nucleate pieces soon regained their specific form. The nucletis 

 is thus indispensable to the preservation of the formative energy of the 

 cell. On general theoretical grounds, and because some of the Protista are 

 enucleate, we are obliged to regard the nucleus as secondary in its origin. It 

 may be that nuclear substance is present in such cells in a diffuse form, and 

 that a later step is the coalescence of the nuclear matter into a single nucleus. 



* Cytokinetic means concerned with movements or phenomena in the cytoplasm. 



