REPORT ON THE HYDROIDA. 
xxxi 
An intermediate form by which the medusoid passes into the fully developed Medusa 
is seen in Pennciria. Here the umbrella has four well-developed radial canals which 
open into a marginal circular canal, while its codonostome, though never attaining the 
widely open condition found in the planoblast, and carrying on its margin only the 
rudiments of tentacles, is sufficiently developed to admit of the free ingress and egress 
of the surrounding water. Notwithstanding, however, this relatively advanced stage of 
development, the gonophore remains permanently attached to the trophosome, and thus 
discharges its generative products without ever becoming free. 1 
Development of the Gonophore . — In the development of the Medusa-bud, the first 
foundation of the Medusa, like that of every bud in the Hydroid colony, consists of an 
outbulging of some definite part of the Hydroid body. There is thus formed a sac- 
like projection whose walls consist of endoderm and ectoderm in direct continuation with 
the same layers in the colony, and whose cavity is also only an extension of that of the 
part from which the bud is given off. 
The subsequent course of the development has recently been studied more especially 
by 0. and E. Hertwig, 2 3 and by Weismann." When the hernia-like sac which forms the 
foundation of the gonophore has attained a certain height the ectoderm of its summit 
becomes thickened and forms an internal prominence, the “ Glockenkern ” or endocodon 
of Weismann. This, pressing on the part of the endoderm which lies beneath it, causes 
an inversion of the endoderm in the form of a cup whose walls, necessarily double as the 
result of the invagination, rise on all sides round the endocodon. In the meantime a 
central cavity has been formed in the solid ectodermal endocodon. This is to become 
the cavity of the umbrella, while by the partial adhesion to one another of the two 
endodermal layers by which it is surrounded, four (or more) longitudinal channels are 
produced. These are the radiating canals, while the intervening endoderm, whose 
lumen has become obliterated by the adhesion, forms the endoderm lamella or vascular 
lamella of the Hertwigs. 
At the same time the bottom of the endodermal cup rises in the form of a hollow 
cone, pushing before it the ectodermal layer which forms the floor of the cavity of the 
endocodon. It thus becomes clothed by this layer, and forms the manubrium of the 
Medusa with its endodermal and ectodermal layer, while the circular canal which in the 
Medusa runs round the margins of the umbrella is formed by intercommunicating 
lateral tubular offsets from the distal extremities of the radial canals. Finally, the 
cavity of the umbrella, still represented by the hollow endocodon, opens externally 
1 Another intermediate form between hedrioblast and planoblast has been recently described by Clarke {Mem. 
Boston Soc. Nat. Hist., vol. iii., 1872) in a Gymnoblastic Hydroid from the American coast. In this, which gives origin 
to medusoids with well-developed radial and circular canals, Clarke has seen some of the medusoids detach themselves 
before liberation of their generative products, while others discharge these without ever becoming free. 
2 0. und R. Hertwig, Das Nervensystem und die Sinnesorgane der Medusen, Leipzig, 1878. 
3 A. Weismann, Die Entstehung, &c. 
