THE PROCESS OF CLEAVAGE. 71 



into equal and unequal, and partial into discoidal and superficial. At the 

 same time HAECKEL endeavoured to derive the different methods of cleavage 

 from one another, and apropos of this directed attention to the important role 

 of the nutritive yolk. 



The processes which take place within the yolk have eluded observation 

 and a correct interpretation even more than the external phenomena of cleav- 

 age, so that it is only in the most recent times that we have acquired a satis- 

 factory insight into them. It is true that the problem, as to what part the 

 nucleus plays in segmentation, has had the uninterrupted attention of investi- 

 gators, but without any solution having been found. For years there were in 

 the literature two opposing views : sometimes one of them, sometimes the 

 other, attained temporarily greater currency. According to one view which 

 was almost universally adopted by the botanists, and was defended on the 

 zoological side principally by REICHERT, and even recently by AUERBACH 

 the nucleus disappears before every division, and is dissolved, to be afterwards 

 formed anew in each daughter-segment; according to the other view the 

 nucleus, on the contrary, is not dissolved, but is constricted, becomes 

 dumb-bell-shaped, and is divided into halves, and thereby induces cell-division. 

 This view was taught especially by such zoologists and anatomists as C. E. 

 v. BAER, JOH. MULLER, KO'LLIKER, LEYDIG, GEGENBAUR, HAECKEL, VAN 

 BENEDEN, and others, who were supported by the observations which they 

 had made on transparent eggs of the lower animals. 



Light was first thrown on the disputed question at the moment when suit- 

 able objects were studied with the aid of higher magnifications, and especially 

 with the employment of modern methods of preparation (fixing and staining 

 reagents). 



The works of FOL, FLEMMING, SCHNEIDER, and AUERBACH on the cleavage 

 of the eggs of various animals mark a noteworthy advance. They still main- 

 tained, it is true, that the nucleus is dissolved at the time of cleavage, but they 

 gave a detailed and accurate description of the striking radiation which arises 

 in the yolk upon the disappearance of the nucleus, and which during the 

 constriction of the egg soon becomes visible in the region of the daughter- 

 nuclei.* SCHNEIDER observed parts of the spindle-stage. 



Soon after this a more exact insight into the complicated and peculiar 

 nuclear changes was obtained by means of three investigations, which were 

 carried out independently and simultaneously on different objects, and were 

 published in rapid succession by BUTSCHLI, STRASBURGER, and the author. 

 It was definitely established by these observations that there is no dissolution 

 of the nucleus at the time of division, but a metamorphosis, such as has been 

 described in the preceding pages. At the same time I likewise proved that the 

 egg-nucleus is not a new formation, but is derived from parts of the germinative 

 vesicle. From this resulted the important doctrine that, just as ill cells, so also 

 all nuclei of the animal organism are derivatives in an uninterrupted sequence, 

 the one from the egg-cell and the other from its nucleus. (Omnis cellula e cellula, 

 omnis nucleus e nucleo.) Through these researches there was furnished for the 



* Eadiating structures had already been observed in the yolk before this, 

 but in an incomplete manner, by different authors by GRUBE in the Hiru- 

 dinea, by DERBES and MEISSNER in the Sea-urchin, by GEGENBAUR in Sagitta, 

 by KROHN, KOWALEVSKY, and KUPPFER in Ascidians, by LEUCKART in Nema- 

 todes, by BALBIANI in Spiders, and by OELLACHER in the Trout. 



