Mosaic Development in Ascidian Eggs. 197 



schon bei schwacher Vergrosserung kenntlich ist; schneidet man 

 also in der Mitte und senkrecht zu dieser Linie, so zerlegt man auch 

 die Bechergastrula in 'vorn' und 'hinten.'" 



It is true that the median plane is marked out by a nearly 

 straight line, though Castle's figures to which Driesch refers show 

 this line between endoderm and not between ectoderm cells, but 

 any one who has studied these embryos knows how difficult it is 

 to determine the median plane in this way, especially in living 

 material. Even in stained and mounted preparations it would 

 not be a sure guide, much less could it be relied upon in the study 

 of living gastrulae. Whether the median plane appears as a 

 straight line or not depends entirely upon whether that plane lies 

 directly in the line of vision, and conversely some of the transverse 

 planes ot cleavage may appear as straight lines if they lie in the 

 line of sight. Thus Fig. 7 shows several transverse rows of ecto- 

 derm cells which in the hinder part of the embryo are curved back 

 in the middle and forward at the sides, but if the embryo were 

 rotated forward so that the polar body were brought to the highest 

 point these transverse rows would appear nearly straight. 



I am convinced therefore that the half gastrulae from which 

 Driesch obtained apparently normal larvae were right or left 

 halves and not anterior and posterior ones as he supposed. 

 Whether these larvae were really normal, i. e., whether they had 

 the organs of both the right and left sides, cannot be determined 

 from Driesch's figures or descriptions, since he seems to have 

 considered that the only evidence required to show that a larva is 

 complete is that it should have a head and a tail. 



The fact that Driesch always obtained partial larvae from the 

 anterior and posterior halves of an elongated gastrula, where the 

 chief axis is unmistakable, requires no comment. 



IV. OTHER EXPERIMENTAL WORK ON THE ASCIDIAN EGG. 



Chabry's ('87) contribution on the normal and teratological 

 embryology of ascidians contains not only the most careful and 

 complete experimental work which has ever been done on the 

 ascidian egg but it is at the same time such an excellent analytical 

 treatment of the normal development that it deserves to rank as 

 an embryological classic. The experimental part of his paper 

 was based upon an unusual knowledge of the normal and patho- 



