542 MATURATION AND IMPREGNATION OF THE OVUM. 



FIG. 17. Ovum of Asterias glacialis, after the coalescence of the male and female 

 pronucleus (copied from Fol). 



shewing the presence of two nuclei before the commencement 

 of segmentation. Biitschli was the earliest to state from ob- 

 servations on Rliabditis dolicJiura that the first segmentation 

 nucleus arose from the fusion of two nuclei, and this was sub- 

 sequently shewn with greater detail for A scans nigrovenosa, by 

 Auerbach (i). Neither of these authors gave at first the correct 

 interpretation of their results. At a later period Biitschli (5) 

 arrived at the conclusion that in a large number of instances 

 (Lymtuzus, Nephelis, Cucullanus, &c.), the nucleus in question 

 was formed by the fusion of two or more nuclei, and Strasburger 

 at first made a similar statement for P/iallusia, though he has 

 since withdrawn it. Though Biitschli's statements depend, as 

 it seems, upon a false interpretation of appearances, he never- 

 theless arrived at a correct view with reference to what occurs 

 in impregnation. Van Beneden (3) described in the rabbit 

 the formation of the original segmentation nucleus from two 

 nuclei, one peripheral and the other central, and he gave it 

 as his hypothetical view that the peripheral nucleus was derived 

 from the spermatic element. It was reserved for Oscar Hertwig 

 (11) to describe in Echinus lividus the entrance of a sperma- 

 tozoon into the egg and the formation from it of the male 

 pronucleus. 



Though there is a general agreement between the most recent 

 observers, Hertwig, Fol, Selenka, Strasburger, &c., as to the 

 main facts connected with the entrance of one spermatozoon into 



