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.).
In the succeeding developmental period the blastopore, which has so far had the form of a circular pore at the posterior extremity of the body, undergoes a series of very remarkable changes. In conjunction with a gradual elongation of the larva it travels to the ventral side, and is prolonged forwards to the velum as a groove. The middle part of the groove is next converted into a tube, which opens externally in front, and posteriorly communicates with the archenteron. The walls of this tube subsequently fuse together, obliterating the lumen, and necessarily causing at the same time the closure of the blastopore. The tube itself becomes thereby converted into a plate of cells on the ventral surface between the epiblast and the hypoblast[106].
While the above changes have been taking place the mesoblast has become established. It is derived from the lateral and ventral cells of the hypoblast.
After the establishment of the germinal layers the further evolution of the larva makes rapid progress. A transverse groove is formed immediately behind the velum, which is especially deep on the ventral surface; and the stomodæum is formed as an invagination of the anterior wall of the deeper section of the groove. Behind the stomodæum the remainder of the ventral surface grows out as a flattened foot.