The external yolk-sack is not vascular, and probably the absorption of the yolk for the nutrition of the embryo can only take place in the internal yolk-sack. The most remarkable feature of the Cephalopod yolk-sack is the fact that it lies on the opposite side of the alimentary tract to the yolk cells, which form a rudimentary yolk-sack in such Gasteropoda as Nassa and Fusus. In these forms, the yolk-sack is at first dorsal, but subsequently is carried by the growth of the liver to the right side. In Cephalopoda on the contrary, the yolk-sack is placed on the ventral side of the body. What is known of the development of the alimentary tract in the Polyplacophora has already been mentioned. In the Lamellibranchiata (Lankester, No. 239), the mesenteron early grows out into two lateral lobes which form the liver, while the part between them forms the stomach. In Pisidium the intestine is formed from the original pedicle of invagination, which remains permanently attached to the epiblast. The stomodæum is formed by the usual epiblastic invagination, and becomes the mouth and oesophagus. The development of the crystalline rod and its sack do not appear to be known. In the adult the sack of the crystalline rod opens into a part of the alimentary tract which appears to belong to the mesenteron. Were however the development to shew them to be really derived from the stomodæum they might be interpreted as rudiments of the organ which constitutes the odontophore and its sack in cephalophorous Mollusca--an interpretation which would be of considerable phylogenetic interest. BIBLIOGRAPHY. _General._ (238) T. H. Huxley. "On the Morphol. of the Cephal. Mollusca." _Phil. Trans._ 1853. (239) E. R. Lankester. "On the developmental history of the Mollusca." _Phil. Trans._ 1875. (240) H. G. Bronn and W. Keferstein. _Die Klassen u. Ordnungen d. Thierreichs_, Vol. III. 1862-1866. _Gasteropoda and Pteropoda._ (241) J. Alder and A. Hancock. "Devel. of Nudibr." _Ann. and Magaz. Nat. Hist._, Vol. XII. 1843. (242) N. Bobretzky. "Studien über die embryonale Entwicklung d. Gasteropoden." _Archiv f. mikr. Anat._, Vol. XIII. (243) W. K. Brooks. "Preliminary Observations on the Development of Marine Gasteropods." _Chesapeake Zoological Laboratory_, Session of 1878. Baltimore, 1879. (244) O. Bütschli. "Entwicklungsgeschichtliche Beiträge (Paludina vivipara)." _Zeit. f. wiss. Zool._, Vol. XXIX. 1877. (245) W. Carpenter. "On the devel. of the embr. of Purpura lapillus." _Trans. Micros. Soc._, 2d series, Vol. III. 1855. (246) W. Carpenter. "On the devel. of the Purpura." _Ann. and Mag. of Nat. Hist._, 2d series, Vol. XX. 1857. (247) E. Claparède. "Anatomie u. Entwickl. der Neritina fluviatilis." Müller's _Archiv_, 1857. (248) H. Eisig. "Beitr. z. Anat. u. Entwickl. der Geschlechtsorg. von Lymnæus." _Zeitschr. f. wiss. Zool._, Vol. XIX. 1869.