XXI.] EMBRYOLOGY. 27?> 



stock from which they have been detached, and are known 

 as Medusae, or jelly-fish. From the ova which they pro- 

 duce, animals are developed, like the parent stem, but 

 unlike the Medusa : and these again produce Medusae. 

 So that we have this metagenesis, which is perhaps the Meta- 

 most beautiful instance of the kind in the whole animal S^^^^i^- 

 kingdom : — 



A. Parent stem, comparable to the leaf-bearing trunk of 

 a plant, producing Meduste by a non-sexual process similar 

 to the formation of buds by a plant. 



B. Medusa, comparable to the flower of a plant, except 

 that it is detached ; producing, by a sexual process, ova 

 that develop into the likeness of the parent stem. 



There is no fundamental distinction between the species 

 in which the flower-like organs mature their products 

 while still in connexion with the parent stem, and those 

 in which they become detached in the form of Medusae. 

 The two ways are observed in allied species ; indeed they 

 sometimes occur in the same species. 



4. An instance has been observed in the genus Lizzia, Medusa 

 and in all probability many more such instances yet jj^g^'^'^™^ 

 remain to be discovered, of Medusae being directly pro- directly. 

 duced by a Medusa, without any metagenesis or meta- 

 morphosis.^ 



This series is evidently a similar one to those which I Parallel 

 have traced through the Batrachians, the Insects, and the ®®"®®- 

 Crustaceans ; and it is, to my mind at least, impossible to 

 doubt that the members of this series, as of the others, 

 have been descended each from the member enumerated 

 before it.^ 



The conclusions arrived at in this chapter may be thus Summary, 

 summed up : — 



The embryonic or larval form of a species is most pro- 

 bably a record of what the ancestral form was from which 

 the species has been derived. 



1 See Dr. Allman's Report on the Reproductive System in tlie Hydroida 

 (Hydrozoa), British Association Reports, Newcastle, 1863. 



2 It is perhaps unlikely that the exact species from which any decidedly 

 unlike species is descended can be still in existence; but very similar 

 ones may be. 



