ORGANIZATION AND CELL-LINEAGE OK A.SCIDIAN EGG. 



J! I 



far as known, they are formed at the ectodermal pole of the egg. His conclusions 

 were stated in the most positive manner and have been widely accepted, notwith- 

 standing that such an orientation is absolutel} unique, and for this very reason 



VIII 



Fig. VII. Four-cell stuni' of Ciona iniestinalis viewed from the animal pole ; the creuated line 

 represents the boundary between the protoplasm and yolk; the dotted line marks the anterior 

 limit of the crescent at the vegetal pole ; the four cells are approximately equal in size. 



Fig. VIII. Four-cell stage of Cynthia partita seen from the animal pole ; the limits of proto- 

 plasm and crescent are represented as in the preceding figure; the two posterior cells are a little 

 smaller than the anterior ones. 



should have been received with caution. Inasmuch as Castle's work is the most 

 thorough and extensive treatment of the early development of ascidians since the 

 appearance of Van Beneden and Julin's paper, and since his conclusions are diamet- 

 rically opposed to my own. it seems desirable to give with some fulness his conclu- 

 sions as to orientation as well as the evidences upon which these conclusions are 

 based. In speaking of Van Beneden anil Julin's work he says ( 1894, p. 200): . . . 

 'It is my purpose to show that by yielding themselves to conjecture in so small a 

 matter as these three cell divisions, the eminent authors fell into an error which 

 invalidates the most important conclusions of their otherwise excellent work. For 

 in correlating the 44-cell stage with the 32-cell stage they have changed the orien- 

 tation so that they have identified the dorsal side of one with the ventral side of 

 the other, the endodermal half of one with the ectodermal half of the other. Their 

 orientation of all the stages prior to the jj-ce/l stage is accordingly wrong. Their 

 terms ectodermal and endodermaL ventral and dorsal, as employed up to this stage, 

 must be interchanged." Again with regard to the point at which the polar bodies 

 form he says (1894, p. 211) : "1 have repeatedly seen the polar bodies and observed 

 continuously the cleavage stages following their formation. These observations lead 

 to the surprising but unavoidable conclusion that the point on the surface of the 

 egg at which the polar bodies form becomes later the center of the dorsal or endo- 

 dermal half of the eps." Again in his later work (1896, p. 22b) he says with 

 regard to this matter: . . '-The form changes accompanying maturation occur, in 

 Ciona at least, and presumably in ascidians in general, at the pole of the egg oppo- 

 site to that at which they occur in Amphioxus, and, so far as known, in all other 

 animals producing eggs with polar differentiation; for the changes connected with 



