2l8 Ed-ium G. Conklin. 



in the direction of cleavage and the consequent closing of the 

 injured side are more apparent in isolated blastomeres of the 2-cell 

 stage than in those of later stages. 



8. Gastrulation. In right or left or anterior halves, gastrula- 

 tion usually proceeds as if the fragment still formed part of the 

 whole; even though the gastrula may be rounded in form the 

 location of the different substances shows that it is strictly partial. 

 Not infrequently isolated blastomeres give rise to exogastrulae, 

 which ultimately right themselves. In posterior halves and in 

 quarter embryos, gastrulation does not at all resemble the normal 

 process, either in methods or results. 



9. Right or Left Half Embryos. A lateral half embryo is 

 usually closed along the injured side; it has a head and a tail; a 

 typical notochord, which is formed only from the chorda cells of 

 the surviving side, and which is therefore composed of half the 

 normal number of cells; an atypical neural plate and sense vesicle, 

 formed only from the typical neural plate cells of the surviving 

 side; a typical mesenchyme area in which the atrial invagination 

 of one side is formed and three typical rows of muscle cells on one 

 side of the notochord, but none along the injured side. In the 

 latest stages to which these lateral embryos were reared (corre- 

 sponding to the period of metamorphosis in normal larvae) the 

 muscle cells have begun to grow around the hinder end of the noto- 

 chord to the side on which they were lacking; but in no case are 

 the three rows of the normal embryo present on this side. Prob- 

 ably only one atrial invagination and one papilla are ever formed 

 in these lateral embryos. These are therefore half embryos in 

 which some cells have grown over from the uninjured to the injured 

 side, but in which absolutely no change has taken place in the 

 potency of the individual cells or of the different ooplasmic sub- 

 stances. 



10. Anterior Half Embryos. Embryos derived from the two 

 anterior quadrants of the egg have no trace of muscle cells nor of 

 muscle substance; although the normal number of chorda cells are 

 present they rarely if ever form a notochord but usually escape 

 from the body of the embryo and lie scattered in the perivitelline 

 space; the neural plate cells are present in normal number and 

 position but the plate rarely, if ever, invaginates to form a sense 

 vesicle; in late stages sense spots are formed from certain cells of 

 the neural plate; cells of the gastral endoderm and general ecto- 



