462 The Larval Theory of the Origin of Cellular Tissue. (May, 
larva, which occurs mouth downwards, as shown by Schultze in 
Sycandra. This primitive opening is also present at an earlier 
‘stage in the free larve of all the higher forms of Carneospongie. 
Haeckel’s parallel between the development of the egg and the 
evolution of forms among Protozoa, from the unnucleated to the 
nucleated, seems to be correct, and the morula can be equally 
closely compared with the colonial free forms of Flagellata like 
Uvella and Volvox. Recent investigations by Balbiani, Bütschli 
and Engelmann have removed almost every obstacle, which for- 
merly existed, to the close homology of the single cell among 
Metazoa with a full-grown adult Protozoön. We can now even 
closely compare the nuclei of the Protozoa and its changes during 
conjugation and subsequent casting off of part of the nucleus, to 
the casting off of polar cells in the egg. The heads of the sperma 
tozoon in the Calcispongiz have been shown, by Poléjaeff, to 
arise from division of a part of the nucleus, the other or central 
part either remaining quiescent or, as this author states, form the 
tails. Almost every author, especially La Vallette St. George 
the most thorough student of this subject, consider all sperm 
tozoa as derived from simple processes of cell division. The ei 
sential and general character of reproduction among Protos 
similar, being, as is well known to observers, fissiparous division 
of the whole body, but this division takes place in the adults > 
the end of a series of transformations, and is not character! 
larval stages. The building up of the tissue in Metazoa 1S due to 
the quick and rapid division of cells, and if it is granted that Meta- 
Zoa may be aggregates of Protozoa, they must be considered, 1 
fact, larval forms descended from Protozoa and inheriting the ten- 
dency to reproduction, by fissiparous division, at an early $ 
In other words, they reproduce while still larvæ and before re 
ing through any transformations, except the very primitive 
which precede and include the nucleated stage. 
We can account for the three layers, by suppos Metaz0a 
must have been among the protozodnal ancestors of me e d 
transitional forms which were colonial, and in which & mi 
the feeding zoöns, the collared and flagellated cells, pe 
stic of 
ing that there 
i i i w 
ferentiated into flattened protective zoons. The next $ sd 
be represented by another form, in which to these two cay ie 
rting 20° 
zoons, a third would be added in the shape of supp? 
and 
We can readily imagine all of these as attached ¢ 
olonies, 
