460 The Larval Theory of the Origin of Cellular Tissue, (May, 
THE LARVAL THEORY OF THE ORIGIN OF CELLU- 
LAR TISSUE. 
BY PROFESSOR ALPHEUS HYATT. 
T a meeting of the Boston Society of Natural History, held 
March 5, Professor Hyatt reviewed the history of investiga- 
tions upon the structure of the Porifera; concluding that though 
true Metazoa, they nevertheless still retained characteristics of 
Protozoa. They possess, in adults, the three typical layers, ec- 
toderm, mesoderm and endoderm, and reproduce by means of 
true nucleated eggs and spermatozoa developed in the mesoderm. 
The embryos in the lower forms develop, as observed by him 
and figured by Barrois in Halisarca, by passing through a hollow 
morula, as do also several species of the higher forms of Carneo- 
spongiz, having silicious spicules, which are now by most al- 
thors supposed to have only a solid morula. 
The differentiation of the egg is slightest in forms like Hali- 
sarca, and possibly also in Gastrophysema, one of the Calcispot- 
giz; if Haeckel’s figures of these really represent sponges. In the 
higher forms of both Carneospongiz and Calcispongie, especially 
the latter, the eggs become differentiated into endoderm and ecto- 
derm at a very early stage of segmentation, and all larval stages 
are much shortened, in accordance with the law of concentration 
and acceleration in development. This law is founded upon the 
fact, now generally established by observation, that the earn ei 
descendant types inherit characteristics and tendencies, or habits 
at earlier stages than those in which they first originated in a 
ancestors.! 
Thus the young of Sycandra is the exact structural parallel 4 
the adult of Leucosolenia, a genus of a lower order, Asan ie 
is at first a tube with thin walls as in Leucosolenia, and et a 
velops the characteristics of its own order, Sycones, pane 
thick wall, and the ccelomatic cavity branches into radia ae 
keeping up connection with the exterior through open paft all 
pores. The cells in the main cavity of the larva, ware 
flagellated and collared, lose their cilia and collars excep a 
the enlargements of the branches occur, forming the S thick 
essential characteristic gastric cavities of all sponge? r 
mesoderm. 
1 See Evolution of Cephalopods, Science, Nos. 52-53» 
alopods, Proc. Bost. Soc. Nat. Hist., Vol. XXII, p. 262. 
ica 
