148 INTERPOLATION OF A CONTINUOUS 



A sort of gamomorphic pullulation may be traced in the 

 very development of the reproductive organs of some medu- 

 soids, for while generally in this family the ova and sper- 

 matozoa are formed directly in the substance of the walls 

 of the manubrium or central polype, in a few species se- 

 condary sacs are budded off as their matrices.* The same 

 idea has suggested itself to Professor AUman, as appears 

 from the following passage iu his Notes on the Hydroid 

 Zoophytes : — " In Laomedea dicJiotoma, L. penicidata, &c., 

 the generative elements are never formed in the manubrium 

 of the Medusa bud, but in peculiar bodies situated on the 

 course of the radiating canals. Now these bodies, at least 

 in the Medusa of L. dickotoma, which I carefully examined 

 with regard to this point, are constructed precisely on the 

 plan of the sporosacs in Clara, Hydractinia, &c. These 

 sporosacs must be viewed as special zooids, representing one 

 term in the ' alternation of generations' of the individual. 

 Just so must the reproductive bodies (sporosacs) which bud 

 from the radiating canals of the Medusa of Laomedea 

 dichotoma, be regarded as special zooids, and as represent- 

 ing a term in the life-series of the Zoophyte. 



" In Eudendrium \atractylis\ ramosum, for example, we 

 * 



vitality of the lower species favours gemmation, in the same degree it 

 tends to confound all the appendages of the body with gemmae. The de- 

 tachment of a part may decide its being a gemma (though not absolutely, 

 as we see in the case of the Hectocotylus), but so long as parts continue 

 iu adhesion, it must often be impossible to determine between a gemma 

 and a hypertrophied appendage, as all parts tend with development of 

 their organization to take on the typical form of the group, which in the 

 case of the Ccelenterata is that of a bell-shaped polype. See in connection 

 Avith this the speculations of M'Cosh on the relation between the vena- 

 tion of a leaf — the vegetable unit or phyton — and the ramifications of the 

 corresponding tree. Typical Forms, Bk. II., ch. 2. 



* Compare Huxley's account of the development of ova and sperma- 

 tozoa in the Hydrozoa generally (Oceanic Hydrozoa, pp. 21-22), and Dr. 

 Wright's remarks on the medusoids of certain species of Laomedea (Edin- 

 burgh Philos. Journal, Jany., 1859, p. 111). 



