REGENERATION IN EGG AND EMBRYO 237 



blastomeres of another ascidian, Molgnla manhattcnsis . He has more 

 fully worked out the cleavage, and finds that the isolated blastomere 

 segments as a part, i.e. as it would have segmented had it remained 

 in connection with the rest of the egg. In general appearance 

 the half-cleavage seems to differ from the half of the complete 

 cleavage, because rearrangements of the blastomeres occur, but 

 despite these shiftings the form of the division is always like that of 

 a part. A whole embryo develops, although there may be defects 

 in certain organs, which are due, he suggests, to the smaller amount 

 of material available for the development of the larva. 



Zoja showed in 1894-1895 in a number of jellyfish that the iso- 

 lated blastomeres produce whole larvae of smaller size. 1 In one form, 

 liriope, the endoderm that forms the digestive tract is normally de- 

 laminated at the sixteen-cell stage, each cell of the blastula wall 

 dividing into an inner and an outer part. In the blastula from the 

 one-half blastomere this delamination also takes place when sixteen 

 cells are present, and not at the preceding cleavage when only eight 

 cells are present. In this form, therefore, the whole number of cells 

 develops before the delamination takes place, and the one-half larva 

 is composed of the same number of cells as is the normal embryo at 

 this stage, but the cells are only half as large. In other species the 

 endoderm appears to begin to develop in the half-larvae when only 

 half the total number of cells is present. 



The conditions in the egg of the bony fishes are very different 

 from those in the preceding forms. The protoplasm, from which the 

 embryo is produced, accumulates at one pole to make the blastodisc. 

 After the cleavage of this blastodisc, the blastoderm that has resulted 

 grows over the yolk sphere at the same time that the embryo is form- 

 ing along one meridian. I carried out some experiments, in 1895, 

 on the eggs of Funduliis hetcroclitus. If one of the first two 

 blastomeres of the egg of fundulus is destroyed, the remaining one 

 produces a whole embryo. If three of the first four blastomeres are 

 removed, the remaining one may produce a whole embryo of small 

 size. The problem of development is, in the case of the fis-h, different 

 from the other cases described, inasmuch as the whole yolk sphere is 

 left attached to the remaining blastomere and is covered over by cells 

 derived from this blastomere. The smaller embryo that is formed 

 lies on a yolk of full size. 2 



Wilson's work on amphioxus has been already described in con- 



1 Bunting ('94) also found that isolated blastomeres of hydractinia make whole em- 

 bryos. 



2 If the yolk of the dividing egg is partially withdrawn without disturbing the blasto- 

 meres, the form of the cleavage may be altered, but a normal whole embryo develops over 

 the smaller yolk sphere. 



