MUSEUM OF COMPARATIVE ZOOLOGY. 67 
dissimilar in origin as the central nervous system and the alimentary 
tract usually are, it becomes a matter of no little importance to solve 
the two problems, what is the origin of these growing regions, and what 
that of the two layers. Through the works of Barrois (86), Ostroumoff 
(’87), Vigelius (88), and especially Prouho (90), on the metamorphosis 
of the larva and formation of the first polypide of Gymnolemata we 
are fairly well acquainted with the facts in this group; but a careful 
study has not heretofore been made of the Phylactolaemata with reference 
to the points mentioned above. Korotneff (’89) and Jullien (90) have 
published quite extensive papers on the ontogeny of Phylactolemata, 
which describe too incompletely the stages which should reveal the 
required facts. 
In order to throw a little light on these questions, I undertook the 
study of the embryology of two species of Phylactolemata. But before 
beginning the account of what I have found, it is necessary to remind 
the reader of some facts concerning the origin of the polypides in the 
adult colonies. For our knowledge of these we are chiefly indebted to 
Braem (’90, pp. 18-32) ; it has also been my privilege to confirm many 
of them. 
The details of the budding process are slightly different in Plumatella 
and Cristatella. In the latter genus the body wall becomes highly mod- 
ified as it grows older by the formation of secreted masses which nearly 
fill most of the ectodermal cells. In Plumatella, on the contrary, the 
ectodermal cells retain, for the most part, a more primitive, unmodified 
condition. Here, moreover, by a rapid growth at the neck of the pol- 
ypides, the individuals are carried to considerable distances from one 
another, whereas in Cristatella there is a less rapid growth resulting in a 
compact stock. 
In Plumatella, the whole of the embryonic tissue from which any bud 
arises does not go to the formation of a polypide, but a part of it re- 
mains as the neck of the polypide, and gives rise by cell proliferation to 
the body wall and the Anlage of a new bud. Thus the Anlage of each 
bud is part of that of a preceding bud. The question remains yet un- 
solved, Whence came the Anlage of the first polypide? Since the em- 
bryonic tissue of the inner layer of the bud, which seems to take the 
most active part in the formation of the bud, gives rise to both the lining 
of the alimentary tract and the wall of the brain, it becomes an ex- 
ceedingly interesting question, From what germ layer is this inner bud 
layer derived ? 
In Cristatella, as in Plumatella, not all of the embryonic tissue from 
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