THE NUCLEI OF UROLEPTUS MOBILIS 



323 



there is no internal structural characteristic to distinguish it from 

 the stationary pronucleus, a peculiar external or cytoplasmic 

 structure now appears which is decidedly characteristic and 

 which is absent from the stationary nucleus. This structure 

 consists of a clearly defined homogeneous mass of very fine 



Figs. 65, 6G, and 67 Three pairs with migrating pronuclei in different positions. 

 The attraction sphere precedes the pronucleus. X 800. 



granules which replace the ordinary alveolar make-up of the 

 cytoplasm. It resembles an attraction sphere and behaves as 

 such in the later history (figs. 64 to 66 and 71 to 75). 



The wandering pronucleus in each cell, preceded by this homo- 

 geneous granular mass, moves towards the anterior end where cell 

 fusion has taken place (figs. 64, 65, 66, 71 to 75). The two pass 



THE JOURNAL OF EXPEKIMENTAL Z' OLOGY, VOL. 27, NO. 3 



