DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
727 
The polymorphism of the Pennatulids was first observed by Verrill in 1864 in 
Renilla , and was afterwards shown by Kolliker to be of general occurrence in the 
group. It occurs also in some of the Alcyonacea, as in Sarcophyton (Moseley) and 
Heteroxenia (Kolliker), and Professor Verrill has informed me of Iris discovery of 
rudimentary individuals in two species of Paragorgia, members of the Gorgonacea. 
For our knowledge of the embryology of the Alcyonaria we are almost entirely 
indebted to Kowalevsky’s well-known researches, though Lacaze-Duthiers, many 
years earlier, made a few observations on the development of Corallium. In 1873 
Kowalevsky gave some account of the embryological development of Alcyonium 
digitatum and Gorgonia verrucosa* * * § and in 187.9 published a brief account of the 
early development of Sympodiuni coralloides and Clavularia crassci, t which he studied 
in conjunction with Marion. Kolliker has given a brief account of the develop¬ 
ment of the buds in Halisceptrum, and Dalyell published fragmentary notes on 
the early development of Virgularia,\ 
Even less is known in regard to the development of the colony in the Pennatulacea. 
Fritz Muller observed in 1864 the simple axial polyp of Renilla and gave a few 
notes upon its structure. Kolliker figures a very young colony of Pteroicles and 
gives a few notes concerning the young stages of Kophobelemnon. Willemoes-Suhm 
has also described and figured one or two of the early stages of the colony in 
Umbellularia.§ A thorough study of the mode of budding has, however, never been 
made; and the observations just mentioned, though of interest, are too incomplete to 
be of great value. 
In all of the Alcyonaria thus far studied the germ-layers appear to be differentiated 
through some process of delamination. Among other polyps, however—as we know 
from the observations of Kowalevsky, Lacaze-Duthiers, Metschnikoff, Jourdan 
and others—some forms undoubtedly pass through a typical invaginate gastrula 
stage, while others appear to develop as delaminate planulse. Balfour states on 
the authority of Kleinenberg,|| that in a number of Zoantharia the segmentation 
is unequal, “indicating, perhaps, the occurrence of an epibolic gastrula.” I shall, 
however, show further on, that inequality in cleavage is by no means a certain 
indication of epibolic invagination. 
With this brief sketch of the anatomy and embryology of the Alcyonaria, in which 
* “ Untersuchungen liber die Entwickelung der Coelenteraten, Nacbricbten der Kaiserl., Gesellsch. der 
Freunde der Naturkenntniss der Antliropologie und Ethnographic.” Moskau, 1873 (Russian). Abstract 
in Hoffmann and Schwalbe’s ‘ Jahresbericht,’ 1875, Bd. ii. 
t ‘ Zoologischer Anzeiger,’ No. 38, 1879. 
+ ‘Rare and Remarkable Animals of Scotland,’ vol. ii., pp. 181-190, t. Kolliker. 
§ ‘Annals and Magazine of Natural History,’ vol. xv., 1875. 
|| ‘Comparative Embryology,’ vol. i. 
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