ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 17 



of indirect cell division. Inasmuch as sonic of the cleavages also throw lighl on 

 this problem, its further consideration will be postponed to the section which deals 

 with the first clca\ age. 



5. Movements of Spindle and of Nuclear Plasm ; formation of Polar Bodies. 



In Cynthia the tirst maturation spindle and the surrounding nuclear plasm re- 

 main indefinitely in the condition shown in figures 77 and 78 unless the egg be ferti- 

 lized. In Ctona the stage at which the maturation processes come to rest is a little 

 more advanced than in Cynthia, as is shown in figure 17li: the peripheral layer of 

 protoplasm is here collected over the lower hemisphere of the egg, and the nuclear 

 plasm which has escaped from the germinal vesicle forms a layer over the entire 

 upper hemisphere. Unfertilized eggs may remain in this condition for at least 

 three or four hours and still he capable of fertilization and normal development ; 

 Imt if they remain unfertilized for ten or twelve hours the nuclear plasm spreads 



IP.S. -' p S. 



1P.S. 



Fig. I. Section of an egg of Cynthia partita which had lain twelve hours without being fertilized. 

 The first polar spindle (1 p. -. lies in tin- position in which it was first formed ; the peripheral layer 

 of yellow protoplasm ip. l.i remains uniformly distributed over the surface, hut the clear protoplasm 

 has spread throughout the yolk and broken it up into irregular masses (compare with figs. 77 and 78 

 showing unfertilized est:s in normal condition). 



Fig. II. Stained preparation of an entire egg of Cynthia partita, showing small spindles at oppo- 

 site poles (1. p. s.i. which are possibly two first maturation spindles, though more probably one of these 

 is a precociously developed sperm spindle. 



through the substance of the yolk in irregular masses {cf. text lig. I), and the eggs 

 thereafter are not capable of normal fertilization. The maturation and further 

 development of the egg are finally and forever halted in this early stage unless 

 the eo-g be fertilized. As soon as a spermatozoon enters the egg active move- 

 ments of the protoplasm begin and a localization of different ooplasmic materials 

 occurs, which will be described later; at the same time the fust maturation spindle 

 moves to the animal pole and is turned from a paratangential to a nearly radial 

 position. The daughter chromosomes then separate and the first polar body is 

 extruded (figs. 66-68 and 79-82). 



The second maturation spindle is smaller than the first, as Castle has shown. 

 and. like the first, is paratangential in position in early stages and only later 

 becomes radial. The second polar bod) is extruded close to or immediately under 



:; JOTJBN. a. N. S. PHILA., Vol. XIII 



