[2] Von Baer who is often stated to have established the above generalization really maintained a somewhat different view. He held (Ueber Entwickelungsgeschichte d. Thiere, p. 224) that the embryos of higher forms never resembled the adult stages of lower forms but merely the embryos of such forms. Von Baer was mistaken in thus absolutely limiting the generalization, but his statement is much more nearly true than a definite statement of the exact similarity of the embryos of higher forms to the adults of lower ones.
[3] Huxley was the first to shew that the body of the Cœlenterata was formed of two layers, and to identify these with the two primary germinal layers of the Vertebrata.
[4] The value of these identifications as well as of those below is discussed in its appropriate place in the body of the work. Their citation here is not to be regarded as necessarily implying my acceptance of them.
[5] Dicyema, if it is a true Metazoon, would seem to form an exception to this rule.
[6] In the vegetable kingdom there are numerous types of Thallophytes, which throw a considerable amount of light on the relation between sexual reproduction and conjugation. Subjoined are a few of the more striking cases. In Pandorina at the time of sexual reproduction the cells which constitute a colony divide each into sixteen, and the products of their division are set free. Pairs of them then conjugate and permanently fuse. After a resting stage the protoplasm is set free from its envelope after division into two or four parts. Each of these then divides into sixteen coherent cells and constitutes a new Pandorina colony. In Œdogonium the fertilization is effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere (oospore) then undergoes segmentation like the ovum of an animal; but the segments, instead of uniting to form a single organism, separate from each other, and each of them gives rise to a fresh individual (swarm-spore) which grows into a perfect Œdogonium. In Coleochæte the impregnation and segmentation take place nearly as in Œdogonium, but the segments remain united together, acquire definite cell walls, and form a single embryo. There is in fact in Coleochæte a true sexual reproduction of the ordinary type. (Vide S. H. Vines “On alternation of generation in the Thallophytes.” Journal of Botany, Nov., 1879.)
[7] The case of Pyrosoma, which might be cited in this connection, is probably secondary.
[8] For an excellent account of this subject, vide Allen Thompson’s article Ovum in Todd’s Cyclopædia. The metamorphosis of the Echinoderms included under this head in Thompson’s article is now known not to be a proper case of alternations of generations.
[9] The appearance of Vertebrata on the globe as the forms which most frequently preyed on Invertebrate forms, and were themselves not so liable to be devoured, has no doubt had a great influence on the metamorphosis of internal parasites, and has amongst other things resulted in these parasites usually reaching their sexual state in a vertebrate host.
[10] For details vide Chapter on Insecta.
[11] The distinction drawn by Huxley between ova and pseudova does not appear to me a convenient one in practice.