352 



LOUIS I. DUBLIN. 



stitution of the nuclei. In the highly-modified " liver-cells," in 

 the epithelium of the gut (Fig. 4), in the cells of the atrium and 

 tentacles (Fig. 5), the primitive conditions are alike preserved. 



From the universality of the occurrence of this phenomenon, it 

 is difficult to escape the conviction that we are here concerned 

 with conditions of considerable importance in the ultimate con- 

 stitution of the nucleus. We must now inquire what light the 

 fertilization and the early cleavage stages will throw on the prob- 

 lem ; for it is here that the constitution of the nucleus ought 

 most readily to be made out. In the first place, the two pro- 

 nuclei do not completely fuse in the act of fertilization. In this 

 regard, Pcdicellina. recalls the condition found by many observers 

 in other forms. Indeed, the individual chromosomes are often 

 formed in both pronuclei before their apposition has occurred and 

 in no case is there much, if any possibility of their fusion during 



FIG. 6. 



FIG. 7. 



fertilization. The nuclei are extremely large and apparently cor- 

 related with this is a fact that each contains many nucleoli ; the 

 number varying from two to as many as seven or more ( Fig. 

 6). In the 2- (Fig. 7), 4-, and 8-cell stages, the same condi- 

 tions with reference to the nucleoli exist. As the cleavages go 

 on a little further the conditions become more in accordance 

 with what was observed in the latter somatic stages. The nuclei 

 become successively reduced in number and it is no uncommon 

 occurrence to find in a i6-cell stage as many as half the cells 

 with two nucleoli ( Fig. 8 ). These two may fuse into one, as is 

 the case in the oogonia and spermatogonia already described. In 

 the 32-64-cell stage, the mono- or bi-nucleolate condition is 

 already the all-prevailing one, all the stages in the approxima- 

 tion and fusion of the two into one being present ( Fig. 9 ). From 

 this point forward, I have never found, in any of the many em- 



