446 MORGAN. [Vol. X. 



is made up of fewer cells than on the right side, and the cells 

 are individually larger on the left side, but not sufficiently so 

 to make equal the right and left mesodermal masses. The 

 notochord and nerve-cord are apparently normal and bilateral. 

 The sections show, also, that the defective germ-ring is com- 

 posed of loose rounded cells, with a few irregular cells beneath. 



Now it is possible in this embryo that the same cause that pro- 

 duced the defect in the germ-ring also produced a defective 

 mesoderm on the same side, and that there is no cause and 

 effect between these two phenomena, but both are due to the 

 same cause. The sections, however, taken in connection with 

 all of the preceding evidence, give one the impression that the 

 small amount of mesoderm of the left side is due to the absence 

 of the germ-ring on that side. 



A study of these abnormal forms leads to the same conclu- 

 sion arrived at from a study of the normal development. The 

 embryos drawn in Figs. 32 and 33 point definitely to the con- 

 clusion that the mesoderm of the germ-ring is the same in part 

 as the mesoderm of the embryo. The dilute solution does not 

 seem to affect this layer. On the other hand, these same embryos 

 show that the middle axial plate of cells is destroyed or injured 

 by the solution. When this plate is destroyed or injured, 

 the posterior elongation of the embryo is likewise modified. 

 Emb^-yos like those of Figs. 35 and 36 show, moreover, that 

 the axial plate may elongate in the posterior direction inde- 

 pendently of the germ-ring, and extend beyond it ; while 

 Figs. 37 and 38 show very conclusively that the embryo may 

 elongate posteriorly independently of the germ-ring of one side. 

 The experimental evidence, as gathered from a large number 

 of observations, of which Figs. 36 and 40 are illustrations, 

 points to the same conclusion, viz., the possibility of a nearly 

 symmetrical axial elongation independent of the germ-ring. 



Historical and Critical Review. 



It is not my intention to enter into a long historical account 

 of the various theories of the method of formation of the fish- 

 embryo. I shall attempt to touch only on those points that 

 seem to me to be essential. 



