86 T. H. MORGAN. 



appear to be more or less stratified in the order just given. This 

 is best seen in the inverted type, in which the ectoderm turns in 

 at the top. The mesoderm forms a sheet of cells wrapped around 

 the neural plate, and the endoderm is drawn upwards as a sheet 

 of cells over the mesoderm. (See Fig. I, PI. XXIV., in my 

 paper of I9O3). 1 The endodermal layer, that is drawn upwards, 

 appears to be that which is normally turned in around the lips 

 of the circular blastopores. In addition to this there is always 

 formed a tube opening on the surface that is lined by yolk cells. 

 The opening of this tube lies near the equator of the egg and 

 forms a crescent-shaped mouth there. The cells that line this 

 tube correspond, I think, to those that make the anterior end of 

 the normal archenteron. This part of the archenteron of the 

 normal embryo appears to develop by the drawing apart of the 

 yolk cells. 



The mesoderm of the lithium larv?e is made up of the cells 

 that lie just beneath the ectodermal cap. If we may draw any 

 conclusion from the condition of the mesoderm in the lithium 

 forms in regard to the origin of the first formed mesoderm of the 

 normal embryo, we would conclude that it does not come from 

 those cells of the 32- to 64-cell stages that are drawn beneath the 

 surface at the top of the egg, but from cells lower down at about 

 the level of the upper portions of the lower four blastomeres of 

 the eight-cell stage. There may be, however, not a little latitude 

 in respect to the potency of many of these cells, and their loca- 

 tion is probably also a factor in their differentiation. 



EFFECTS OF LITHIUM CHLORID AND SODIUM CHLORID 

 ACTING TOGETHER. 



The effect of lithium seems to depend in large part upon chem- 

 ical, rather than upon osmotic, effects. Other salts also act 

 chemically in somewhat the same way as lithium, but to a less 

 degree. I have tried the effects of these salts acting together and 

 also separately on the eggs of Rana palustris and of Bnfo loitigi- 

 nosits. The material for the former species I owe to Dr. Stevens. 



A solution of LiCl 0.5 and NaCl 0.6 per cent, gave the results 

 shown in Figs. 32 and 33. The cleavage was not only greatly 



1 AV/M'.f An-/iir, XVI. 



