Development of the germinal layers, pp. [544‑553]. Development of the larval appendages and metamorphosis, pp. [553‑573]. Summary and general considerations, pp. [573‑576].
Chapter XXI. Enteropneusta. Pp. [579‑583].
Index. Pp. [584‑590].
Appendix.
EMBRYOLOGY.
INTRODUCTION.
Embryology forms a large and important department of Biology. Strictly interpreted according to the meaning of the word, it ought to deal with the growth and structure of organisms during their development within the egg membranes, before they are capable of leading an independent existence. Modern investigations have however shewn that such a limitation of the science would have a purely artificial character, and the term Embryology is now employed to cover the anatomy and physiology of the organism during the whole period included between its first coming into being and its attainment of the adult state.
The subject-matter of the science of Embryology admits of a twofold classification. It may be placed under a series of heads, each dealing either with a special group of organisms, or with a special department of the whole science. If classified in the first of these ways the science will naturally be divided into an Embryology of Plants, and an Embryology of Animals; each of which admits of further subdivision. In the second way the subject falls under two primary heads; viz. Physiological Embryology and Anatomical Embryology.
The present treatise deals only with the Embryology of Animals, and is further confined to those animals known as Metazoa. The science is moreover treated from the morphological or anatomical, rather than from the physiological side.