MUSEUM OF COMPARATIVE ZOOLOGY. 285 



gain circular outlines while they migrate toward the poles of the figure. 

 They are the new nuclei. They reach or pass the centre of the new 

 cleavage spheres, but do not reach the swollen end of the karyolytic 

 figure. The latter meantime gradually disappears. First the stem 

 becomes slimmer; the rays become shorter, and then disappear; the 

 centre of the sun becomes flattened to the form of a disk or a meniscus 

 lens concave toward the cleavage plane ; the stem disappears ; the me- 

 niscus becomes thinner, and also disappears. According to Auerbach, 

 the new nuclei are formed by the re-collection of the diffused nuclear 

 sap into a single drop for each sphere ; but inasmuch as each of these 

 is larger than the half of the old nucleus, additional nuclear sap must 

 have been extracted from the protoplasm. 



It did not fail to impress Auerbach as peculiar that the formation of 

 the new nuclei should be accompanied by their motion in substantially 

 the same direction as that which prevails during the dissolution of the 

 old nucleus, instead of the opposite direction ; but it does not seem to 

 have caused him any misgivings as to the accuracy of his theoretical 

 propositions. 



With Ascaris and Strongylus during segmentation the nuclei never 

 acquire a membrane, and for this reason the membrane, when it does 

 exist, must be considered as a secondary structure produced by a conden- 

 sation of the layer of yolk protoplasm immediately enveloping the nuclear 

 fluid. The nucleoli arise in the nucleus after it has come to rest, not 

 before.* In regard to the exact manner of their origin, Auerbach in so 

 far modifies the opinion held in the first part of his paper as to admit 

 that they are not necessarily portions of the surrounding protoplasm which 

 are subsequently detached and set free in the fluid of the nucleus, but 

 that molecules of protoplasm may have been detached with the formation 

 of the nuclear fluid, and have remained distributed through it till they at 

 length became visible by becoming grouped into the observed nucleoli. 

 The author is also less confident that a multinucleolar condition always 

 arises by the repeated division of a single original nucleolus. 



This method of nuclear increase, to use Auerbach's own words, "ent- 

 spricht in der Hauptsache der einerseits von Reichert, andererseits von 

 den neueren Phytologen aufgestellten Lehre. Aber es ist ausgezeichnet 

 dadurch, dass die Substanz des aufgelosten alten Kerns nicht in dem 

 ganzen Zellenleibe sich tertheilt, sondern in einem beschrankten inneren, 

 eigenthiimlich gestalteten, doppelt gegliederten und durch strahlige Fort- 



* In the case of the pronuclei, it will be seen that Auerbach says the nucleoli arise 

 before the former execute their migratory motion. 



