ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 29 
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 absolutely unique, and for this very reason 
үн! 
G. VII.—Four-cell stage of Ciona intestinalis viewed from the animal pole; the crenated line 
Жым the boundary between the protoplasm and yolk; the dotted line marks 0 the нен ta 
. VIII.—Four-cell stage of Cynthia partita seen from the animal pole; the in its of proto- 
plasm and crescent are represented as in е preceding figure; the two apad бузам cells are a little 
me es 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. Іп speaking of Van Beneden and Julin’s work he says (1894, р. 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. | ZXezr 
orientation of all the stages prior to the 44-cell stage zs accordingly wrong. Their 
terms ectodermal and endodermal, ventral and dorsal, аз 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) :— 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 dorsa/ or endo- 
dermal half of the egg.” Again in his later work (1896, p. 226) he says with 
regard to this matter:—. . “Тһе 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 Amphzoxus, and, so far as known, in all other 
animals producing eggs with polar differentiation ; for the changes connected with 
