The yolk-sack of the Cephalopoda. The yolk, as has already been stated, becomes at an early period completely enclosed in a membrane formed of flattened cells, which constitutes a definite yolk-sack. It is, in the more typical forms of Cephalopoda, divided into an external and an internal section, of which the former is probably a special differentiation of the median part of the foot of other cephalophorous Mollusca (vide p. [272]). At no period does the yolk-sack communicate with the alimentary tract. The two sections of the yolk-sack are at first not separated by a constriction. In the second half of embryonic life the condition of the yolk-sack undergoes considerable changes. The internal part grows greatly in size at the expense of the external, and the latter diminishes very rapidly and becomes constricted off from the internal part of the sack, with which it remains connected by a narrow vitelline duct.
The internal yolk-sack becomes divided into three sections: a dilated section in the head, a narrow section in the neck, and an enormously developed portion in the mantle region. It is the latter part which mainly grows at the expense of the external yolk-sack. It gives off at its dorsal end two lobes, which pass round and embrace the lower part of the œsophagus. The passage of the yolk from the external to the internal yolk-sack is probably largely due to the contractions of the former.
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 œsophagus. 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.