OVUM. 



31 



observations of previous inquirers, as well as 

 the addition of new facts, from which an 

 almost entire certainty is acquired that the 

 various phenomena do actually succeed each 

 other in the order above stated, and that the 

 occurrence of alternate or intermediate gene- 

 rations in these animals is established. 



Von Siebold had in 1835 described in the 

 Monostomum mutabile the development of the 

 first embryo from the ovum in the Gregarina- 

 like or animalcular form, and had shown the 

 next change to consist in the formation within 

 the first embryo of a second body endowed 

 with locomotive power, and independent vita- 

 lity, and differing both from its immediate 

 parent and from the acKilt.* V. Siebold, as well 

 as others, had ascertained the Cercariae to be 

 themselves incomplete animals, and to proceed 

 from others by a process of internal production 

 of a non-sexual kind. Steenstrup therefore di- 

 rected his attention particularly to trace these 

 Cercariae on the one hand, in their development 

 into complete Distomata, and on the other, 

 backwards through their progenitors towards 

 the first origin from an ovum. His observa- 

 tions were made principally in three kinds of 

 Cercaria, which, along with their antecedent 

 and succeeding conditions, are found in great 

 numbers in the fresh water snails, Lymneus 

 stagnalis, Paludina vivipara, Planorbis, &c., 

 and which had been previously named Cer- 

 caria echinata, C. armata, and C. ephemera. 

 In these, especially in the first, the conversion 

 of an encysted Cercaria by metamorphosis 

 into a Distoma, and the descent of the Cercaria 

 (by metagenesis) through two progenitors, 

 not themselves Distomata, was ascertained, 

 but he did not succeed in tracing these bodies 

 back to their origin from ova. By a com- 

 parison, however, of the body formed within 

 the animalcular embryo of the ovum of the 

 Monostomum mutabile, as observed by V. 

 Siebold, with the first progenitor of the Cerca- 

 ria, to which it was found to present a remark- 

 able similarity, the chain of evidence seemed 

 to be complete, and Steenstrup found himself 

 in a position to announce the general views 

 of alternate generation, which have ever since 

 their first publication attracted the greatest 

 attention, and contributed in a powerful de- 

 gree to modify and direct the investigation of 

 the generative processes in the lower animals. 



To the immediate progenitor of the Cercaria 

 Steenstrup gave the name of nurse (altrix, 

 Amme), in allusion to its nursing or nourishing 

 function, and to the immediate progenitor of 

 this one he gave the appellation of " parent or 

 grand-nurse." These terms may be objection- 

 able, but an unnecessary amount of criticism 

 seems to have been bestowed on them by 

 some writers. They are adopted hypotheti- 

 cally by Steenstrup ; they do not appear to 

 withdraw him from the matter-of-fact state- 

 ment of his observations; and they seem to 

 be, in many respects, short and convenient 

 terms in the description of the phenomena. 

 These bodies have in the Cercaria echinata all 

 the appearance of distinct animals, that is, a 

 * See Wiegmann's Archiv, 1835. 



body with a head separated by a neck or col- 

 lar, a tail or caudal projection, and two pro- 

 cesses of the integument similar to limbs, a 

 mouth and alimentary cavity, and they move 

 with all the appearance of spontaneity ; but 

 it ought to be remarked that the form and 

 powers of these nursing or formative cases 

 differ considerably in various other species, 

 and in some present so little of the external 

 form or endowments of an independent ani- 

 mal, that the more general appellations of 

 germ-cases, or germ-sacs, or sporo-cysts, may 

 be more appropriate to them.* 



It is chiefly among the aquatic Gastero- 

 pod Mollusca, and a few land ones, that these 

 observations have been made ; but V. Siebold 

 has extended them to some of the Trematoda 

 inhabiting the air-sacs and other parts of 

 water fowls, which no doubt come from the 

 same Mollusca, and obtain access to the seat 

 of their final parasitic habitation from the 

 water or along with food, into which they 

 have come as Cercaria?, after having previously 

 been parasitic in the Mollusca. It is easy to 

 understand how the ova of the Distomata 

 discharged from the bodies of the water 

 fowl may gain their place in the Mollusca. 

 V. Siebold has observed in a very interesting 

 manner also the passage of the Cercariae 

 into the bodies of water insects (larvae of 

 Ephemera and Perlida), which he placed 

 together with a quantity of Lymneus stag- 

 nalis, from the various parts of whose bodies 

 the Cercariae were discharged in numbers out 

 of their nursing capsules : the penetration of 

 the integument of the insect by the Cercaria 

 and the mode of casting its tail being precisely 

 the same as that observed by Steenstrup in the 

 Mollusca.f 



Both these observers agree that the first 

 and second germ-cases (or nurses), and the 

 Cercariae, or Distoma-larvae, arise by a process 

 of gradual development from extremely minute 

 granular spherules, which are at first situated 

 in the posterior region of the body, or between 

 the alimentary cavity and the integument. 

 These are certainly not ova : but we are at a 

 loss to state to what class of reproductive 

 germs they may be referred with greatest 

 accuracy.:}: 



It is known that the bodies which inhabit 

 the aqueous chamber of the eyes of many 

 fishes are imperfect Distomata. Steenstrup 

 has frequently observed these larvae in the 

 pupa state adhering to the inside, and some- 

 times to the outside, of the cornea, and he has 

 occasionally noticed a delicate streak through 

 the cornea, indicating the track through which 

 the animal has penetrated ; and he considers 

 it as extremely probable that all the Trema- 

 toda of the eyes of fishes, of which a vast 

 variety has been described by Nordmann$, are 



* See Victor Carus, iiber den Generations -wechsel, 

 for a figure of these more simple forms of sporo- 

 cysts. 



t See the Article PARASITES, in R. Wagner's 

 Handworterbuch der Physiologic. 



t See Fig. 28. </. 



Mikographische Beitrage, &c., 1832. 



