ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 397 
logue of this part. Agassiz, in his account of Tubularia Couthouyi, Agass., 
ignores the existence of any membrane between the well-developed mesotheca 
of this species and the generative mass which surrounds the spadix. In 
Tubularia indivisa, however, this membrane cannot be overlooked, especially 
in the male, though in the female it would seem to disappear at an early 
period, and may thus escape detection. 
The Phanerocodonic Gonophore.—The phanerocodonic gonophore shows 
itself, at first, in every case as a minute hernia (fig. 15 A), consisting of endo- 
derm and ectoderm, and having its cavity in free communication with that of 
the gonoblastidium or of the trophosome from which it springs, thus in no 
respect differing at this period from the corresponding stage in the develop- 
ment of the adelocodonic gonophore, or indeed in that of a polypite branch. 
It is very difficult to follow satisfactorily the several steps by which this 
primordial tubercle becomes ultimately converted into a medusa. T have 
bestowed great attention on it in different species of Hyprorpa, and have 
more recently subjected the development of the medusa-bud in Corymorpha 
nutans to a very laborious examination, which has led me to adopt the pro- 
cess now about to be described, as the true interpretation of the phenomena 
presented in this hydroid. 
We first find that four equidistant processes (fig. 15 B c), consisting of endo- 
derm and ectoderm, with an included cavity, which is a continuation of that of 
the hernia-like tubercle just mentioned, have begun to grow upwards from a 
Fig. 15.—Development of medusa in Corymorpha nutans. 
A, very early stage of medusa-bud when it presents the form of a simple hernia-like 
tubercle whose cavity is in communication with the somatic cavity of the hydroid; B, 
more advanced stage; C, stage still further advanced; D, transverse section of C; E,a 
stage still more advanced than C; F, transverse section of RE. 
a, ectoderm ; 6, endoderm ; a', more external of the two layers into which the ectoderm 
of the bud has split; a", the more internal of these two layers ; c, radiating gastrovascular 
canals ; d, manubrium. 
