OF ARTS AND SCIENCES. 51 



comitantly with the formation and expansion of the cleavage-cavity. 

 From this stage onward the history of the periblast is the history 

 of the cleavage of the marginal cells. Our success in tracing this 

 history is largely due to the methods employed. It was the differen- 

 tial staining before mentioned that first placed us on the right track. 



One or two points insisted on by Hoffmann deserve notice in this 

 connection. Hoffmann's ideas respecting the manner in which the 

 cleavage of the teleostean ovum is introduced were fully anticipated by 

 E. van Beneden, in 1878.* "Directly after fecundation," says Bene- 

 den, " the egg of the Osseous Fish divides into two very unequal cells, 

 very dissimilar, differing in constitution and significance ; the one is 

 the germ which segments and from which the blastodisc is derived ; the 

 other is formed by the deutoplasmic globe, clothed, at least partially, 

 by a thin layer of protoplasm forming ' the intermediate layer.' " 

 (No. 4, p. 54.) Hoffmann brings, in addition to his observations, the 

 following a priori consideration to the support of this view: — 



" Wenn das Ei der Knochenfische eine Zelle ist, woriiber man wohl 

 nicht mehr streiten wird, dann ist es auch ganz natiirlich, dass bei der 

 eintretenden Furchung, bei der ersten Theiking in Archiblast und 

 Parablast, der erste Furchungskern die Theilung einleitet, sonst 

 wurde hier der Fall vorliegen, dass eine Zelle sich theilte, ohne dass 

 der Kern sich daran betheiligte und in dem einen Stiick unverandert 

 liegen blieb, wahreud das andere Stiick kernlos wurde." (No. 13, 

 p. 126.) 



These two citations are sufficient for the present to show precisely 

 how this matter stood atthe beginning of our investigations. The 

 evidence produced by Hoffmann in favor of his and Van Beneden's 

 view is of a very positive character, and can only be impeached by 

 calling in question the accuracy of his observation. He claims to 

 have seen the first cleavage-spindle in a vertical position, and that the 

 first cleavage-plane takes place accordingly in a horizontal direction. 

 Just after the division of the spindle, two nuclei were seen in the axis 

 of the "Keim"; one lying iu the floor near the vitellus, the other 

 at some distance from this and directly above it. The subsequent 

 division of the upper nucleus was accompanied by cleavage of the 

 archiblast; but the division of the inferior nucleus was not attended 

 by cleavage, and led simply to the formation of a multi-nucleated cell, 

 the so-called parablast. The parablastic nuclei were seen in each of 

 the subsequent stages of cleavage, and kept equal pace with the archi- 



* Cf. KupfEer on Laichen und Entwicklungsgeschichte des Ostseeherings. 



