PHILOSOPHICAL TRANSACTIONS. 
I. Contributions to the Developmental History of the Mollusca. By E. Rat Lankester, 
M.A., Fellow of Exeter College , Oxford. Communicated by George Rolleston, 
F.R.S . , Linacre Professor of Physiology. 
Received January 19, — Read March 12, 1874. 
No. I. The Early Development of Pisidium pusillum. 
In the months of April and May 1871 I obtained a supply of the freshwater bivalve 
Pisidium pusillum, from a muddy little stream near Jena, in Saxe Weimar. 
The fact that the development of the eggs of this mollusk takes place within a pair 
of brood-cavities formed at the root of the inner gill-lamella on each side, enables the 
observer very readily to obtain embryos in different stages of development. 
Leydig and O. Schmidt had previously to this described the development of species 
of Cyclas, which genus really embraces Pisidium. Leydig studied Cyclas cornea , 
Schmidt studied Cyclas calyculata. At the period when their studies were made the 
questions of histogenesis were not quite in the same position as they are to-day ; and 
accordingly in their papers little will be found relating to the topics now discussed. 
Moreover, on account of the greater transparency of the eggs of Pisidium , I have been 
able to work at them with an objective of as high magnifying-power as Hartnack’s 
No. 10 a immersion. 
I propose in the present communication to take up the figures in the accompanying 
Plates in the order in which they present themselves as developmental stages, and thus to 
combine an account of the changes and their significance with a description of the Plates. 
Plate 1. fig. 1 represents an ovum from the oviduct of Pisidium pusillum at the 
breeding-season. The egg-cell is not yet fully grown, and is seen to lie in close appo- 
sition to a coiled highly refringent mass, which is a secretion of adjacent cells, and is 
assimilated by the egg-cell as “ deutoplasm,” in consequence of which its “ body,” which 
is now pellucid, becomes granular, as seen in fig. 3 *. 
Plate 1. fig. 2 represents a number of such ova with adjacent masses of deutoplasm 
and spermatozoa. The genus Cyclas is hermaphrodite, and so is Pisidium. Whether 
* March 7th, 1875. — I should prefer to speak of such matter uniformly as “food-material” in all eggs 
where it occurs. 
MDCCCLXXY. 
