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 Gymnolsemata we 

 are fairly well acquainted with the facts in this group; but a careful 

 study has not heretofore been made of the Phylactolsemata with reference 

 to the points mentioned above. Korotneflf ('89) and Jullien ('90) have 

 published quite extensive papers on the ontogeny of Phylactolsemata, 

 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 Phylactolsemata. 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 



