126 



F. M. BALFOUK, 



entered^ it gradually travels towards the female pronucleus. This 

 latter, around wbicli the protoplasm no longer has a radial 



Fig. 15. — Ovum of Asterias glacialis, with male and f(Miialc pronucleus and 

 a radial striation of the protoplasm around the former. From living 

 ovum (copied from Fol). 



arrangement, remains motionless till it comes in contact with the 

 rays of the male pronucleus, after which its condition of repose 

 is exchanged for one of activity, and it rapidly approaches the 

 male pronucleus, and eventually fuses with it (Fig. 16). 



ijip^ 





is?- 





■i^ 



Fig. 16. — Three successive stages in the coalescence of the male and female 

 pronucleus in Asterias glacialis. From the living ovum (copied 

 from Fol). 



The product of this fusion forms the first segmentation nucleus 

 (Fig. 17), which soon, however, divides into the two nuclei of the 

 two first segmentation spheres. While the two pronuclei are 

 approaching one another the protoplasm of the egg exhibits 

 amoeboid movements. 



Of the earlier observations on this subject there need perhaps 

 only be cited one of E. van Beneden, on the rabbit's ovum, 

 showing the presence of two nuclei before the commencement of 

 segmentation. Biitschli was the earliest to state from observa- 

 tions on Rhahditls dolichura that the first segmentation nucleus 

 arose from the fusion of two nuclei, and this was subsequently 

 shown with greater detail for Ascar'is nigrovenosa, by Auerbach (1 ). 

 Neither of these authors gave at first the correct interpretation 

 of their results. At a later period Biitschli (5)arrivedat the con- 



