Unequal segmentation takes place in some of the cyclostoma and in the oldest fishes in just the same way as in most of the amphibia. Among the cyclostoma (“round-mouthed”) the familiar lampreys are particularly interesting. In respect of organisation and development they are half-way between the acrania (lancelet) and the lowest real fishes (_Selachii_); hence I divided the group of the cyclostoma in 1886 from the real fishes with which they were formerly associated, and formed of them a special class of vertebrates. The ovum-segmentation in our common river-lamprey (_Petromyzon fluviatilis_) was described by Max Schultze in 1856, and afterwards by Scott (1882) and Goette (1890).
Unequal total segmentation follows the same lines in the oldest fishes, the selachii and ganoids, which are directly descended from the cyclostoma. The primitive fishes (_Selachii_), which we must regard as the ancestral group of the true fishes, were generally considered, until a short time ago, to be discoblastic. It was not until the beginning of the twentieth century that Bashford Dean made the important discovery in Japan that one of the oldest living fishes of the shark type (_Cestracion japonicus_) has the same total unequal segmentation as the amphiblastic plated fishes (_ganoides_).[22] This is particularly interesting in connection with our subject, because the few remaining survivors of this division, which was so numerous in paleozoic times, exhibit three different types of gastrulation.
[22] Bashford Dean, _Holoblastic Cleavage in the Egg of a Shark, Cestracion japonicus Macleay. Annotationes zoologicae japonenses,_ vol. iv, Tokio, 1901.
Fig. 48 Sagittal section of the gastrula of the water-salamander. Fig. 48—Sagittal section of the gastrula of the water-salamander (_Triton_). (From _Hertwig._) Letters as in Fig. 47; except—_p_ yelk-stopper, _ mk_ beginning of the middle germinal layer.)
The oldest and most conservative forms of the modern ganoids are the scaly sturgeons (Sturiones), plated fishes of great evolutionary importance, the eggs of which are eaten as caviar; their cleavage is not essentially different from that of the lampreys and the amphibia. On the other hand, the most modern of the plated fishes, the beautifully scaled bony pike of the North American rivers (Lepidosteus), approaches the osseous fishes, and is discoblastic like them. A third genus (Amia) is midway between the sturgeons and the latter.