1. Plerocercoid larvae of the pseudophyllidean cestode Ligula intestinalis from the body cavity of roach, were cultured in vitro at 40°C. in a variety of saline and nutrient media. About 65% of such cultures were aseptic.

2. During cultivation, larvae produced acid by-products (unidentified) and the pH fell rapidly.

3. The presence of these acid by-products slowed down development, or, if present in sufficient quantity, caused death.

4. In order to obtain development in nutrient media in a period (3 days) comparable to that required in a bird (the normal host) it was necessary to renew the medium 24-hourly.

5. 6% of the eggs produced from a worm cultured in horse serum were fertile. Fertile eggs were never obtained from larvae cultured in any other media.

6. Certain bacterial infections had no apparent detrimental effect on development, but others were toxic.

7. Some larvae underwent development in non-nutrient medium (¾ strength Locke's solution). The exact conditions under which this occurred was not determined.

8. Fragments (3 cm. long), of larvae or larvae with either scolex or posterior half removed, underwent development to the stage of oviposition in nutrient media.

9. Histochemical examination revealed that the plerocercoid larvae were almost fat-free. During cultivation, very large quantities of cytoplasmic fat were produced the quantity being proportional to the duration of cultivation. Fat was produced even under starvation conditions (i.e. during cultivation in saline) and can be considered a metabolic by-product.

10. The fresh plerocercoid contained great quantities of glycogen in the parenchyma and muscle regions. After cultivation in nutrient or saline media, considerable quantities were still present.

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