EMBRYOLOGY OF CLEPSINE. 249 



origin and relation to each other have remained unknown up to 

 the most recent times. It would hardly be profitable to give 

 here the long list of cases which I have catalogued, in which 

 some of these bodies have, possibly, probably, or certainly, been 

 observed, but entirely misapprehended ; 1 shall therefore refer 

 only to the more important of those bearing a recent date. 



The first accurate account of the origin of the cleavage-nucleus 

 was given in 18 74 by Auerbach (vs^^ttt)- This author saw 

 two nuclear bodies arise at opposite poles of the egg 

 (Ascaris and Strongylus), m each of which two or three nucleoli 

 soon appeared. These two bodies approached, and, coming in 

 contact with each other near the centre of the ovum, performed 

 a rotation of 90°, on an axis passing through the median point 

 of contact, perpendicular to the longituduial axis of the egg. 

 A complete fusion of these two pronuclei followed this rotary 

 movement — thus giving rise to the cleavage-nucleus (" central 

 nucleus"). Btitschli (toV t) published as early as 1873 an 

 account of the same bodies, seen in the egg of Ehabditis doli- 

 chura ; but he was uncertain whether the pronuclei arose in- 

 dependently of each other, or by division of a single nucleus, and 

 left it equally undecided in regard to their complete union. 

 Early in 1875 appeared a preliminary communication from 

 Biitschli (#„t)j containing results obtained in the study of 

 several genera of nematoids (Tylenclms, Cepliolobus, Pthabditis, 

 Diplogaster, Cucullanus) and two mollusca (Lymnaeus, Succinea). 

 This communication confirmed the statements of Auerbach in 

 regard to the coalescence of the pronuclei, but raised a doubt in 

 regard to their normal number, which according to Auerbach is 

 two, an opinion confirmed by the latest papers of Fu\ and Hertwig. 



In the same year the beautiful work of Strasburger was 

 published, in which the cleavage-nucleus (" Keirakern ") i" 

 represented as arising from two pronuclei (i-f4, Phallusia mamiJ 

 lata). Shortly after (1875) came the very important investiga- 

 tions of Oscar Hertwig on TorojJneustes lividits. Auerbach 

 (^f-^) had already compared the union of the projiuclei with the 

 " Copulation ziveier Individidiien, odtr weniystens zweier Zellen,'* 

 for the purpose of propagation ; but it was reserved for Hertwisj 

 to show, with little less than positive evidence, that the two 

 pronuclei {" Spermakern," and '* Eikern ") represent male and 

 emale elements, and the fact was thus distinctly formulated ( ry^): 

 *' Der unmittelbar vor der Eurchung in der Eizelle vorhandene 

 einfache Kern, urn welchen die Dolterkornchen in Eadien ange- 

 ordnet sind, ist aus der Cojnilatinn zweier Kerne hervogegangen." 



Almost simultaneously with He^rtwig's paper, appeared the not 

 less important preliminary communication from van Beneden 

 (10), on the maturation, fecuudalion, and development of the 



