OVUM. 



35 



These phenomena are confined to the In- 

 vertebratetl animals, and among them are to 

 be regarded as occasional and exceptional 

 rather than general. Among the Protozoa they 

 may exist to a greater extent than is yet known ; 

 they occur more or less in all the divisions of 

 the Radiata,but more constantly and obviously 

 in the Polypinaand Acalepha than in the Echi- 

 nodermata : among the Mollnsca they have 

 been observed only in the comparatively 

 limited classes of Bryozoa and Tunicata: they 

 are almost universal in two divisions of the 

 class Entozoa, viz. the Cestoidea and Trema- 

 toda, but they are altogether absent in the 

 Nematoidea ; they belong only to the lowest 

 division of the Annelida, and among the Arti- 

 culata proper the almost entirely exceptional 

 examples are confined to the class of insects. 

 Nor are the phenomena universal in those 

 classes or orders of the animal kingdom in 

 which they have been described. In some 

 of these divisions, therefore, and in all the 

 others of the animal kingdom, the manner of 

 the reproductive process is ascertained to be 

 direct, that is, by generation from sexual 

 parents, and by development, or in some in- 

 stances by metamorphosis, from ova to sexual 

 individuals ; to which may be added, in some 

 animals, by the multiplication of like indi- 

 viduals, separate or aggregated, by gemmation. 

 But although this general statement is ap- 

 plicable to the present state of our knowledge, 

 we are still too imperfectly acquainted with 

 many forms of the reproductive process to 

 warrant us in affirming that there ir.ay not yet 

 be discovered many other deviations from 

 that which has long been familiar to our minds 

 as the more common and direct connection 

 between the ovum and the perfect or sexual 

 individuals which produce it. In the mean 

 time let us endeavour, according to our present 

 information, to determine the true character of 

 the seemingly exceptional phenomena, of which 

 the more important have been noticed in the 

 preceding section. 



The views first suggested by Steenstrup, in 

 1842, in connection with these phenomena 

 are unquestionably to be regarded in the 

 light of a discovery, and the attempt to de- 

 prive them of their character of novelty and 

 importance, or to refer them to other pre- 

 viously known general laws, has entirely 

 failed.* It may be that the smaller portion 

 only of the facts on which the views are 

 founded may have been first observed by 

 Steenstrup, and that parts also of these views 

 may be in themselves premature, speculative, 

 or erroneous ; but all discoveries in science are 

 the result of the successive and concurrent 

 observations of a number of inquirers; and no 

 one who has had an opportunity of studying 

 the history of the progress of research into 

 the reproductive function during the last 

 twenty years, will deny that the view which 

 has associated together the phenomena in 

 question under a common principle, has had 



* See Ed. Forbes' Remarks, in Treatise on Naked- 

 eyed Medusas, 1848, p. 83., et seq. 



a most important influence in modifying the 

 doctrines and in guiding and suggesting the 

 researches of those physiologists who have 

 devoted themselves to inquiries into the 

 origin, early conditions, and zoological affi- 

 nities of the lower animals ; and it may truly 

 be affirmed, that no one could at present 

 enter upon the investigation of any obscure or 

 doubtful department of animal or vegetable 

 production among the lower series of these 

 kingdoms of nature, without making especial 

 reference to the views embodied in Steen- 

 strup's generalisation. 



The occurrence of non-sexual multiplication 

 among some of the Invertebrated animals had 

 long been known, as of Polypes, by budding, 

 so admirably described by Trembley in his 

 work published in 1744 ; and of the Aphides, 

 by internal production, discovered by Reaumur 

 and Bonnet ; and of the Nais and Nereis, 

 by external extension, described by Otto F. 

 Miiller, in 1800; the imperfect conditions of 

 some of the Entozoa had been detected by 

 Nitsch and V. Baer in 1818: the two forms 

 of the Salpae were known to Chamisso in 1819, 

 who, more than any other observer, appears 

 to have foreseen in these animals the discovery 

 of alternate or dissimilar generations ; but the 

 first observations, from which the peculiar 

 nature and general doctrine of the phenomena 

 now under consideration may be regarded as 

 having been deduced, are those of Sars, on the 

 compound Polype stock of the Medusae, 

 in 1828, and of Rud. Wagner, on the produc- 

 tion of Medusiform bodies from a polype 

 (Coryne aculeata), in 1833. From that time, 

 observations followed one another in rapid 

 succession, and continue still to crowd upon 

 us, so as to have changed almost entirely the 

 aspect in which we have been accustomed to 

 regard the development of the lower animals : 

 and it is only necessary to mention collec- 

 tively the names of some of those whose 

 observations have contributed most to extend 

 and to establish these discoveries in different 

 classes of animals, to exhibit the importance 

 attached to them by zoologists, comparative 

 anatomists, and physiologists of the highest 

 character in our time, such as Dalyell, Lister, 

 Sars, Loven, V. Siebold, Nordmann, Esch- 

 richt, Steenstrup, Van Beneden, Kolliker, 

 Owen, Dujardin, Milne Edwards, Blanchard, 

 Quatrefages, J Reid, J. Miiller, E. Forbes, 

 Desor, Vogt, Agassiz, Huxley. 



The peculiar nature of these phenomena as 

 compared with those of the better known and 

 more common form of sexual generation, con- 

 sists in this, that in some animals, in all of 

 which, be it observed, the permanence of the 

 species is secured by the sexual production of 

 ova, the body or individual which is developed 

 immediately from the ovum is not, in general, 

 itself the bearer of the sexual organs, but, 

 nevertheless, maintains for a time an inde- 

 pendent existence, or presents the structural 

 and functional characters of a separate or dis- 

 tinct individual ; these characters often dif- 

 fering remarkably from those of the sexual 



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