60 MORGAN. [Vor. IX. 
The reproductive gonads appeared in earlier stages but the 
full account was left till the present. Zhe gonad ts formed 
from the mesoderm. At the posterior end of the gill-region and 
closely connected with the ectodermal wall forming the posterior 
limit of the region the gonads are developed. 
An early stage in the development of a gonad 1s shown in 
Fig. 79 where the nuclei of the mesodermal wall have begun to 
enlarge. The preparation is not entirely satisfactory owing to 
a shrinkage of the mesodermal wall. This is the earliest stage 
of the gonad that I have found. A later stage is shown in 
Fig. 80, where the number of nuclei in the gonad is greater and 
the organ is much larger than before. 
In the oldest stage the gonads have considerably enlarged as 
shown in Fig. 81, drawn to the same scale as the two preceding. 
A central cavity is forming in the center of the reproductive 
cells, but as yet the gonad is not connected with the ectoderm. 
There can be no doubt that the reproductive cells come from 
the mesoderm and not from the ectoderm as Bateson thought 
probable. The gonad is formed as a thickening of the wall of 
the body-cavity where the somatic and splanchnic layers are 
continuous with one another. The ectodermal connection of 
the gonad must come in later. 
CONCLUSION. 
Growth. 
The general phenomena of growth as shown by Tornaria 
present a remarkable series of changes. The larva during the 
early period of its life is suspended in the water and unusually 
free from all external agencies that might modify its structure 
in this way or that. During the earlier stages an immense in- 
crease in size takes place. The ectoderm that forms a single 
layer of cells over the greater portion of the surface has its 
area many times increased. This is accomplished by the cells 
increasing in number. 
The cavity of the blastocoel space increases greatly. The 
digestive tract does not increase to nearly the same extent as do 
the walls of the body, and the result is that the space between 
