284 The American Naturalist. [March, 
There results a solid morula. Serial sections demonstrate that at one 
pole of the morula the cells sink inwards, while the peripheral cells 
grow over, enclosing a cavity within one end of the egg. 
this process is a process of growth of cells around one pole of the 
egg, or whether we have here a process between epibolic and embolic 
gastrulation, cannot be definitely decided. The cells soon begin to 
differentiate into tissues, and only the inner ones retain the yolk 
spherules. The outer layer becomes columnar ciliated ectoderm, 
Those cells lining the enclosed cavity become flattened at several 
places, and push out into passages ending in ciliated chambers. Later 
these latter form the inhalent passages. The remaining cells filling the 
egg contain yolk, and are the so-called mesoderm cells. Some of 
these form needles, each needle the product of a single cell, and by 
their growth push out the ectoderm before them. These changes have 
taken place while the larva was within the sponge tissues ; but it now 
becomes free and swims about with the pole containing the cavity 
directed forwards. The method of swimming described by Götte, 
with the pole containing the cavity directed upwards, is undoubtedly 
pathological. The larval life lasts about twelve hours, —never so long 
as twenty-four. The best observations on the method of fixation were 
made with the horizontal microscope. The larva fixes itself by the 
pole which was directed forwards in swimming,—that is, the end 
containing the gastric cavity. The cavity itself diminishes. The 
young sponge flattens to a crust. The high, cylindrical, ectodermal 
cells become more cubical, then flatten till their longest diameter is 
tangential to the surface. At first the cilia, one to each cell, were 
close together, but as the cells flatten they lie farther apart. The 
above process of fixation and flattening lasts about one-half to three- 
quarters of an hour. The ectoderm cells around the periphery of the 
young sponge begin to spread out over the support to which the 
sponge is fixed, and it takes place by the amceboid-like migrations of 
the peripheral ectoderm cells. The ectoderm is never thrown off, as _ 
Götte supposed, and it seems probable that owing to rough treatment 
of the embryos they lost their delicate ectoderm. After the fixation 
of the larva the ciliated chambers—evagination from the inner 
cavity—come nearer to the surface, fuse with the ectoderm, and form 
the inhalent orifices. The exhalent orifice originates through & 
secondary connection of the inner cavity with the outer world. 
Descensus Testiculorum.—Under the above title Dr. Herman? 
Klaatsch, of Heidelberg, has given, in the Morphologisches Jahrbuch, : 
