6 Prof. Allman on the Hydroida. 



I obtained, however, by means of the towing-net, in the 

 neighbourhood of the locality which produced the Corymorpha, 

 a little Medusa regarding which there can be no doubt that it 

 is a more advanced stage of the medusoid of our Corymorpha, 

 with which in all essential points it is identical. It was about 

 four times the size of the newly liberated medusoid ; and the 

 tentacle had proportionately increased in length, and presented 

 upwards of forty spherules, while the radiating canals at their 

 origin curved upwards towards the summit more decidedly than 

 in the younger form. The generative elements (not yet, how- 

 ever, fully developed, and apparently male, though, from their 

 immature condition, no active spermatozoa could be detected) were 

 very distinctly visible as a pale yellow mass between the endo- 

 derm and ectoderm of the manubrium, which was rendered tumid 

 by their presence. In all other respects the little Medusa was 

 identical with the younger ones, and continued to present the 

 acuminated summit, which was even still traversed by the canal 

 which originally maintained a communication between the tube 

 of the supporting stalk and the cavity of the manubrium. 



Development of the Medusoid-bud. — The medusoid first shows 

 itself as a minute tubercle consisting of ectoderm and endoderm, 

 and containing a simple diverticulum from the cavity of the 

 supporting stalk. It is very difficult to follow satisfactorily the 

 several steps by which this primordial tubercle becomes ulti- 

 mately converted into the complete medusoid ; but if I am right 

 in my interpretation of the appearances which I have observed 

 during a laborious examination of the bud in its different stages, 

 the steps of its development would seem to be as follows : — 



First, a differentiation takes place in the ectoderm of the 

 summit or distal portion of the bud, by which this layer becomes 

 here divided into two lamina?, an outer and an inner — the latter 

 remaining adherent to the endoderm. 



Next, the cavity of the bud extends itself upwards in the form 

 of four thick csecal processes occupying a peripheral position 

 and placed symmetrically round the axis. They are composed 

 of the endodermal lining of the bud-cavity, having outside of it 

 the general ectoderm of the bud ; while the inner of the two 

 lamina? into which this ectoderm splits at the summit extends 

 itself downwards as the processes continue to elongate, and in- 

 vests them on the inner side, or that turned towards the axis of 

 the bud, the space between the two lamina? becoming at the 

 same time larger and larger, and ultimately forming the cavity 

 of the nectocalyx. 



The four processes are destined to form the four radiating 

 canals of the medusoid; and shortly after their first appearance 

 another csecal process has begun to grow up between them, in 



