MUSCLES OF CERATODUS B35 



columns in the first and second branchial segments, which 

 consist of yolk-laden cells, are not continuous with the myotome 

 above, but are continuous with the pericardial wall below. 

 I interpret them as lateral plates. In an embryo of 10-5 mm. 

 (Text-figs. 6-9) the pericardium has retreated a little, and its 

 anterior end is 56 /i behind the thyroid, just in front of the lower 

 end of the first branchial segment. In this segment is the 

 first branchial muscle-plate, the Jow^er end of which is detached 

 from the pericardial wall. In the second branchial segment 

 is the second branchial muscle-plate, the lower end of which 

 is continuous with the pericardial wail. The difference between 

 the two segments is owing to the slight retardation in develop- 

 ment from before backwards — from segment to segment. 

 I use the term * muscle-plate ' to denote those cells of the 

 lateral plate which are obviously muscle-cells and the primordia 

 of the branchial muscles. Though still containing yolk-granules 

 they are distinguishable from the other cells of the lateral plate. 



In explanation of the figures it should be added that in the 

 9-5 mm. embryos the branchial arches slope downwards and 

 slightly backwards, in the 10-5 and 12 mm. embryos they are 

 vertical, in the 16 mm. embryo they slope downwards and 

 forwards. 



I thus fail to find any continuity between the myotome above 

 and the cell-columns in the first and second branchial segments 

 in a 9-5 mm. embryo, i.e. at a stage when, according to Greil, 

 such a continuity exists. The same is true of a 9 mm. embryo. 

 Further, in a 10-5 mm. embryo what is obviously the second 

 branchial muscle-plate is continuous with the pericardial wall. 

 The difference in length of these embryos are so slight that it 

 is improbable that — as is demanded by Greil's theory — what is 

 ' lateral-plate ' in the 9-5 mm. embryo is replaced by down- 

 growth from myotome in the 10-5 mm. embryo. Again, Greil's 

 theory fails to explain why a muscle-plate derived from myo- 

 tome downgrowth should ever be continuous with the pericardial 

 wall. I also fail to find any differences in the shape of the cell- 

 nuclei between the upper and lower parts of the cell-columns 

 of these segments in the 9-5 mm. embryo, as is stated by Greil. 



