When laid the egg is about 0.105 mm. in diameter. It is invested by a delicate membrane, and is somewhat opaque owing to the presence of yolk granules, which are however uniformly distributed through it, and proportionately less numerous than in the ova of most Chordata. Impregnation is external and the segmentation is nearly regular (fig. 1). A small segmentation cavity is visible at the stage with four segments, and increases during the remainder of the segmentation; till at the close (fig. 1 E) the embryo consists of a blastosphere formed of a single layer of cells enclosing a large segmentation cavity. One side of the blastosphere next becomes invaginated, and during the process the embryo becomes ciliated, and commences to rotate. The cells forming the invaginated layer become gradually more columnar than the remaining cells, and constitute the hypoblast; and a structural distinction between the epiblast and hypoblast is thus established. In the course of the invagination the segmentation cavity becomes gradually obliterated, and the embryo first assumes a cup-shaped form with a wide blastopore, but soon becomes elongated, while the communication of the archenteron, or cavity of invagination, with the exterior is reduced to a small blastopore (fig. 2 A), placed at the pole of the long axis which the subsequent development shews to be _the hinder end of the embryo_. The blastopore is often known in other Chordata as the anus of Rusconi. Before the invagination is completed the larva throws off the egg-membrane, and commences to lead a free existence. [FIG. 2. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) The parts in black with white lines are epiblastic; the shaded parts are hypoblastic. A. Gastrula stage in optical section. B. Slightly later stage after the neural plate _np_ has become differentiated, seen as a transparent object from the dorsal side. C. Lateral view of a slightly older larva in optical section. D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (_no_) in front. E. Older larva seen as a transparent object from the side. _bl._ blastopore (which becomes in D the neurenteric canal); _ne._ neurenteric canal; _np._ neural or medullary plate; _no._ anterior opening of neural canal; _ch._ notochord; _so^I_, _so^{II}._ first and second mesoblastic somites.] Up to this stage the larva, although it has acquired a cylindrical elongated form, has only the structure of a simple two-layered gastrula; but the changes which next take place give rise on the one hand to the formation of the central nervous system, and on the other to the formation of the notochord and mesoblastic somites[2]. The former structure is developed from the epiblast and the two latter from the hypoblast. [2] The protovertebræ of most embryologists will be spoken of as mesoblastic somites.