I/O Edzuin G. Conklin. 



cells were found on both sides of the notochord. Indeed, I am 

 not at all sure that this extension of the muscle cells around the 

 end of the notochord is accompanied by any increase whatever 

 in the number of muscle cells or in the number of rows of cells. 

 The latest stage in which I can positively identify the three rows 

 of muscle cells is shown in Fig. 36. In this larva the muscle rows 

 lie nearer the ventral side than in normal larvae (see Fig. 12), and 

 they are evidently extending over the ventral surface toward the 

 opposite side. In later stages the muscle cells become much 

 elongated, but I have not been able to determine the number of 

 rows present. I have found it still more difficult to decide whether 

 the trunk mesenchyme ever extends over to the side on which it 

 was originally lacking, but I believe that this takes place only to a 

 limited extent, if at all, and that Chabry was right when he affirmed 

 that only one atrial invagination is formed in these right or left half 

 embryos. 



2. Three-Quarter Embryos (Figs. 34-35)- 



In connection with the right or left half embryos I shall here 

 consider three-quarter embryos, which, of course, include the 

 whole of the right or left half. Two such embryos are shown in 

 Figs. 34 and 35. In the former the left anterior quadrant was 

 killed in the 8-cell stage; in the latter the right posterior quadrant 

 in the 4-cell stage. The embryo in which the cells of the anterior 

 quadrants were uninjured (Fig. 35) is perfectly normal in its 

 anterior half; its posterior half, however, lacks those parts which 

 would have developed from the cell which was injured. This 

 embryo is younger than the one shown in Fig. 34 and no sense 

 spots are present, but the sense vesicle is closing in a normal 

 manner. This figure well shows that a part of the trunk mesen- 

 chyme is derived from the anterior quadrants, and indeed from 

 the pair of cells A^^, Fig. 6, while a portion of it comes from the 

 posterior quadrants, as may be seen by comparing the right and 

 left sides of Fig. 35. The muscle cells are entirely lacking on the 

 right side, the substance which would have formed them being 

 located in the injured cell B^; they are shown growing around the 

 end of the notochord as in the half embryo shown in Fig. 33. 

 The notochord and nerve tube are apparently full sized, which is 

 explained by the fact that they come from the anterior quadrants, 

 but owing to the lack of the right side of the tail they are somewhat 

 distorted in form. 



