2o6 Edwin G. Conklin. 



yellow, protoplasm in front of the muscle rows on each side of the 

 tail; these areas may be recognized in the early cleavage stages 

 and in no case are both these mesenchyme areas present in right 

 or left half embryos. It is almost certain, therefore, that only one 

 atrial invagination is formed in such embryos. 



We find, therefore, that those parts of the larva which normally 

 lie on the right side are missing in a left half embryo and those 

 which normally lie on the left side are not found in a right half 

 embryo, whereas unpaired organs which lie along the median 

 plane are represented in both lateral half embryos. This is 

 exactly what might be expected from a study of the organization 

 of the egg since the substances, which give rise to median organs, 

 are found along the median plane in both right and left blasto- 

 meres, whereas the materials which give rise to organs of the right 

 side are found only in the right blastomere, those which give rise 

 to organs of the left side, in corresponding positions in the left 

 blastomere. 



Neither Driesch nor Crampton attempt to show that a larva 

 from the right half of an egg has the organs of the left side and this 

 is the whole question at issue; if it does have these organs it is a 

 complete embryo; if it lacks them it is a partial embryo, even if it 

 does have a head and a tail. Chabry found that a larva from one 

 of the first two blastomeres had a head and tail and median organs, 

 but that it did not have the organs of the missing side and this 

 conclusion I can entirely confirm. 



All of the muscle substance (myoplasm) and most of the mesen- 

 chyme (chymoplasm) is localized in the posterior half of the egg, 

 and corresponding to this distribution we find that an anterior 

 half embryo entirely lacks muscles, though it may have a small 

 amount of mesenchyme (that derived from the cell A' ''), whereas 

 a posterior half embryo contains a large number (probably the 

 full normal number) of muscle cells and most of the mesenchyme. 



V. REGULATION IN THE ASCIDIAN EGG AND EMBRYO. 



It is well known from the work of Loeb ('92) and L. Schultze 

 ('99) that the brain of Ciona will be regenerated w^hen extirpated 

 in the adult animal, and that the siphons will be restored when 

 they are cut off. Driesch ('02) has also shown that Clavellina 

 has extraordinary powers of regenerating almost all lost parts. 



