The origin of the mesoderm in the gastropod Viviparus (= Paludina).

For the first account on the development of the mesoderm inViviparus viviparus (L.) [=Paludina vivipara Férussac] we are indebted to Bütschli (1877). According to him the mesoderm arises in other Gastropod from two mesoblastic bands. At first each of these bands consists of only a few cells, but later on the number of cells increases, followed by the breaking up of the two bands and subsequent distribution of the mesoderm cells in the cleavage cavity. In 1891 Erlanger's (1891) exhaustive description of the embryology ofviviparus appeared, in which he made the startling discovery that the cœlom arises as a pocket from the archenteron. This sac is the “primary cœlom”; it becomes bilobed and its connnection is severed with the archenteron. Later on the cells lining it break up and spread irregularly through the segmentation cavity. From the mesenchymatous cells thus produced is developed the pericardium, which represents the “secondary cœlom.” Thus Erlanger was able to bring the Mollusca into line with the Annelida and the Echinodermata with respect to the early developmental stages.