1078 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [98] 



the original structure. This is probably what Lamarck meant in speak- 

 ing of new organs, since the doctrines of special and serial homologies 

 were not developed in his time as they now are. 



The causes or origin of variations are contemplated in this second 

 law, as already urged, especially if we admit that the word need may be 

 replaced by the term sthmdus, as is fully justified by the closing phrases 

 on the intrinsic or reflex actions evoked by a change in the environ- 

 ment. The second law thus becomes very similar in its main features 

 to Huxley's third and provisional statement of the first principles of 

 the doctrine of evolution, as the final alteruative, with which the great- 

 est number of evolutionists would now probably be in accord. 



While we must admit, with Mivart, that the formative forces con- 

 trolling the growth of the embryo from an Qgg seem to be automatic or 

 instinctive, we can assert with some confidence that, in many cases at 

 least, there is positive evidence to the effact that growth force has a 

 determinate direction along a certain axis, and that the first cleavage 

 plane is coincident with the direction of the median long axis of the 

 future animal.* The study of the promorphology of the ovum, as it 

 has been called, thus indicates that the pr jgress of development even 

 during its very early stages exhibits polar phenomena, which are ex- 

 pressed not only as an external polarity, but also as an internal one, 

 involving the most extraordinary rhythmical metamorphosis of the 

 prmordial egg-nucleus and its descendants, which become the nuclei 

 of the cells into which the original germinal mass subdivides. The 

 successive events inv^olved in the ordinary indirect process of segmen- 

 tation seem to be principally the following: 1, the aggregation of 

 the chromatin substance of the nucleus into looped fibers which are 

 freed from the nuclear wall; 2, the radiated arrangement of the seg- 

 ments of chromatin fibers, with their loops directed towards the center 

 of the nucleus and the free ends towards its periphery, 3, the longitu- 

 dinal subdivision of the chromatin fibers into more slender ones, with 

 the same arrangement as before ; 4, systole, or aggregation of the more 

 thickeued chromatin fibers about the incipient ])lane of cleavage, with 

 achromatin fibers forming the poles of the nuclear spindle; 5, diastole 

 of the chromatin fibers, which vsubdivide again and retreat to the nu- 

 clear poles, leaving achromatin fibers intervening between the two polar 

 wreaths which the chromatin fibers form ; 6, completed diastole of the 

 chromatin fibers, which now break up into bead-like masses which be- 

 come applied to the walls of small nucleoplasmic vesicles which are 

 gradually formed at the extreme ends of the j^olar wreaths of chroma- 

 tin fibers; 7, coalescence of the small vesicles lined with chromatin at 

 either end of the spindle to form two new nuclei, which are the centers 

 of the two new cells resulting from the division.! 



* On tlie development of some pelagic fish eggs : preliminary notice. A. Agassiz and 

 C. O. Whitman. Proc. Am. Acad. Arts and Sciences, XX, 1884, pp. 74,75. 



t J. Bellonci : La Caryociuese dans la segmentation de rrouf de I'Axolotl. Arch 

 Italienues de Biologie, VII, 1834, pp. 52-57, 1 pi. 



