66 MORPHOLOGY. 



conical licad, witli a caudal filament of extreme tenuity ; the head is about jjr,r,th of an inch in 

 its longer diameter, and about -j^jth in its shorter. The tail is attached to the wide 

 end. 



In attempting an interpretation of the above appearances, we must, I think, regard the 

 nucleated cells which constitute the contents of the gonopliore in the second of the stages (wood- 

 cut, fig. 31 B) just described as spermatic cells which in the next stage have set free their nuclei ; 

 these nuclei, after liberation from the cells, acquiring a more elongated form, developing a 

 filament, and becoming converted into true spermatozoa. 



Allusion has been just made to the peculiar striated appearance presented by the mature 

 spermatic mass while yet contained within the gonophore. This appearance, wliich is very 

 common in the mature male gonophores of the Hydroida, suggests to us the idea that the 

 corpuscles composing the mass are confined in an exceedingly fine tubular tissue. I have, 

 however, in vain sought for any indubitable evidence of tubes, and I believe that the appearance 

 in question is the result of a mere arrangement of the corpuscles — a condition induced in the 

 plastic mass by the pressure exerted on it by the resisting walls of the gonophore as the mass 

 within increases in volume ; for the component corpuscles have now become changed from the 

 spherical form of the previous stage to a more oval form, and their axes are compelled by the 

 surrounding pressure to take a definite direction. It is a phenomenon which in this view would 

 be purely physical, and which we cannot avoid comparing to that of slaty cleavage, though 

 occurrin<: in an organized and living mass. 



IV. Development. 



There can be no doubt that the phenomena of development, involving as these do the 

 changes of form undergone by the organism in successive periods of time, constitute a department 

 of Morphology, and should, when possible, be treated in connection with other morphological 

 phenomena instead of being included under the head of Physiology, as is the usual, and perhaps 

 in some cases the more convenient, practice. 



In the account already given of various parts of hydroid organisation, it has been found 

 necessary to dwell with more or less detail on their development. A more systematic treatment 

 of hydroid development may now be attempted. This may be best considered under two heads, 

 the one treating of the development of the bud, the other of that of the embryo. 



These two kinds of development, notwithstanding certain differences which necessitate their 

 separate consideration, possess close analogies with one another. The progress of the bud in its 

 development, like that of the embryo, is always from the general to the special; and just as it is 

 impossible at first to point out any difference between embryos which are destined to branch off' 

 into widely separated types, so in their early stages it is impossible to distinguish from one 

 another buds which are destined to become developed into very different forms. Thus, no 

 difference whatever can be detected on their first appearance between three buds, one of which is 

 destined to become a hydranth, another a sporosac, and another a medusa ; and the analogy will 



