On the Structnre and Affinities of Mnestra parasites Krohn etc. 49 



In tlie youiiger specimens of Phyllirhoe^ 7 — 14 millimetres in 

 Icngth, even tliough infested l)y Mnestra^ I could lind none of these 

 pbenomena, but in most (but not in all) of tbe older Pltyllirhoe of 

 20 millimetres and more in lengtb, the "young- embryos" were very 

 abundant, lying in the connective tissue of the mollnsc. Drawings 

 of a few are given upon piate 3. In the early stages the cells 

 are grouped regularly enough. The 2, 4 and 8 celi stages ^ may 

 ali he easily found in certain individuala. At the 16 and 32 celi 

 stages the "young embryos" are for the most part of irregulär 

 contour, ellii)Soidal rather than spherical and with a celi or two 

 projecting beyond the others bere and there and thus marriug the 

 symmetry of the whole. The cells are mostly vacuolated. Furthcr 

 than the 32 celi stage I could not trace the young emlnyos, but 

 in several stages they seemed to divide transversely. 



In certain individuala of Phyllirhoe, there may be noticed locai 

 aggregatious of tiny nuclei which stain well and which I believe 

 may be the spermatozoa of the Mnestra. These nuclear aggrega- 

 tious occurred in the connective tissue in the neighbourhood of and 

 rather posterior to the hermaphrodite glands of the PhyUirhoe. It 

 is supposed that the sperm mother cells of the Mnestra wander to 

 the Position indicated and that the process of spermatogenesis oc- 

 curs there. Fertilisation and the subsequent development would take 

 place within the l)ody of the Phyllirhoe of the embryo. 



Unfortunately I cannot pretend that these conclusions are to 

 be regarded as indisputable facts. The difficulties and possible 

 errors of the correct Interpretation of the phenomena are unusually 

 great. We are justified in regarding the "developing embryos" and 

 "spermatozoa" as being unessential and in a sense foreign to the 

 Phyllirhoe., for they are not to be found in young or in ali old in- 

 dividuals. It is easy to demonstrate that cells having the appearance 

 of Q^^ or sperm mother cells occur in great numbers at the point 

 where the tissues of the Mnesti'a become continuous with the tissues 

 of the Phyllirhoe, but it is not easy to give irrefragable proof that 

 these cells wander to other parts of the Phyllirhoe and there give 

 rise to the "spermatozoi" clumps or to the earliest stages of the 

 "young embryos". 



1 It must be stated that in as much as it is not always easy to enumerate 

 the ex a et number of cells in each embryo the numbers 8, IG, 32 must be 

 taken as approximate only: or merely as descriptive terms to imply that the 

 actual number of cells in the embryo is about that number. 



Mittheilungen a. d. Zool. Station zu Neapel. Bd. 16. 4 



