Dibranchiate pen-sack, as seen in its earliest stage in the embryo _Loligo_, etc., is fused with the surviving remnants of an embryonic shell-gland. When the embryology of _Nautilus pompilius_ is worked out, we shall probably know with some certainty the fate of the Molluscan shell-gland in the group of the Cephalopoda." The funnel. The general development of the funnel has already been sufficiently indicated. The folds of which it is formed are composed both of epiblast and mesoblast. The mesoblast of the anterior part of each half of the funnel would appear to give rise to a muscle passing from the cartilage of the neck to the funnel proper. The posterior parts gradually approximate, but meet in the first instance ventrally. The two folds at first merely form the side of a groove or imperfect tube (fig. 113 C and 124 ff.), but soon the free edges unite and so give rise to a perfect tube, the primitive origin of which by the coalescence of two halves would not be suspected. In Nautilus the two halves remain permanently separate but overlap each other, so as to form a functional tube. [FIG. 116. I. CHITON WOSSNESSENSKII. (After Middendorf.) II. CHITON DISSECTED to shew _o._ the mouth; _g._ the nervous ring; _ao._ the aorta; _c._ the ventricle; _c´._ an auricle; _br._ the left branchiæ; _od._ oviducts. (After Cuvier.) III., IV., V. STAGES OF DEVELOPMENT OF CHITON CINEREUS. (After Lovén.) The figure is taken from Huxley.] Polyplacophora. The external characters of the embryo of Chiton have long been known through the classical observations of Lovén (No. 285), while the formation of the layers and the internal phenomena of development have recently been elucidated by Kowalevsky (No. 284). The eggs are laid in April, May, and June, and are enclosed in a kind of chorion with calcareous protuberances. The segmentation remains regular till sixty-four segments are formed. The cells composing the formative half of the ovum then divide more rapidly than the remainder; there is in this way formed an elongated sphere, half of which is composed of small cells and half of larger cells. In the interior is a small segmentation cavity. From its eventual fate the hemisphere of the smaller cells may be called the anterior pole, and that of the larger cells the posterior. An involution of the cells at the apex of the posterior pole (though not of the whole hemisphere of larger cells) now takes place, and gives rise to the archenteron. At the same time an equatorial double ring of large cells appears on the surface between the two poles, which becomes ciliated and forms the velum. At the apex of the anterior pole a tuft of cilia, or at first a single flagellum, is established (fig. 116 III. and IV.).