Febkuaey 18, 1898.] 



SCIENCE. 



225 



the egg. Each remains in close contact with* 

 its own nucleus, so that there is no possi-f 

 bility of confusing and mistaking them. ' 

 When the pronuclei come together the as- 

 ters also come into contact. The origin of 

 the cleavage centrosomes has not yet been 

 satisfactorily determined. 



In the prophase of the first cleavage 

 the chromatin is clearly distinguishable 

 into two kinds, oxychromatin and basi- 

 chromatin ; the latter only takes part in 

 forming the chromosomes ; the former be- 

 comes arranged like beads on the spindle 

 fibers and is apparently drawn to the two 

 poles. It seems to take no part in the 

 formation of the daughter nuclei and prob- 

 ably forms a part of the granular substance 

 of the sphere. All the cleavage centro- 

 somes undergo a metamorphosis similar to 

 that of the polar spindles and in the tele- 

 phase of each cleavage the poles of the 

 spindle are occupied by a granular sphere 

 frequently as large as the nucleus, or even 

 larger. These spheres, in every case, move 

 to those portions of the cells which lie 

 nearest the polar bodies. In this position 

 they can be recognized through one and, 

 in some cases, two or three subsequent 

 divisions. It results from' the fact that 

 after the first two cleavages the sphere 

 substance is differently distributed to the 

 different cells, the entire sphere substance 

 of one generation always going into those 

 cells of the next generation which lie 

 nearest the animal pole. This differential 

 distribution of the spheres has been fol- 

 lowed through every cleavage up to the 

 24- cell stage. As the form of cleavage is 

 perfectly constant it follows that the sphere 

 substance of any generation goes into cer- 

 tain definite cells which have a perfectly 

 constant origin and destiny. This differ- 

 ential distribution of the spheres is not 

 caused by their specific weight, since their 

 movements are the same in whatever posi- 

 tion the egg may be placed. It seems to be 



ithe result of a form of polarity which, like 

 that of the egg itself, is not the result of 

 gravity. 



The centrosomes do not apparently arise 

 from the sphere substance of the previous 

 division, but some distance from it, and the 

 sphere substance itself never divides, but 

 each sphere ultimately grows ragged at its 

 periphei-y and gradually fades out into the 

 general cytoplasm. 



The differential distribution of these 

 spheres and their subsequent conversion 

 into cytoplasm suggests that they may be 

 important factors in the differentiation of 

 the cleavage cells, and if further investiga- 

 tion should establish the fact that they 

 are in part composed of the oxychromatin 

 of the nucleus it would furnish a basis, in 

 fact, for certain well known speculations of 

 DeVries, Weismann and Roux. 



Considerations on Cell-lineage and Ancestral 

 Reminiscence, based on a Re-examination of 

 Smne Points in the Early Development of An- 

 nelids and Polyclades. Edmdnd B. Wil- 

 son. 



This paper attempted to reconcile the ap- 

 parent contradiction in cell-lineage between 

 the annelids and polyclades, and to show 

 that homology and ancestral reminiscence 

 may appear as clearly in the cleavage 

 period as in other stages. In Leptoplana, a 

 polyclade, all of the first quartets of micro - 

 meres produce ectoblast, as in the annelids 

 or mollusks, while the main mass, if not all, 

 of the mesoblast arises by delamiuation 

 from the second quartet. The formation of 

 ecto- mesoblast ('larval mesenchyme,' or 

 ' secondary mesoblast') from cells of the 

 second or third quartets in the mollusks was 

 interpreted as a reminiscence of what oc- 

 curs in the polyclade, and evidence was 

 given that a similar reminiscence occurs in 

 some annelids {Aricia) . 



In the polyclade the fourth quartet is 

 purely entoblastic ; but the posterior cell 



