Nematoidea. Although the ova of various Nematodes have formed some of the earliest, as well as the most frequent objects of embryological observation, their development is still but very imperfectly known. Both viviparous and oviparous forms are common, and in the case of the oviparous forms the eggs are usually enveloped in a hard shell. The segmentation is total and nearly regular, though the two first segments are often unequal. The relation of the segmentation spheres to the germinal layers is however only satisfactorily established (through the researches of Bütschli (No. 383)) in the case of Cucullanus elegans, a form parasitic in the Perch[154]. [154] The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus. The early development of this embryo takes place within the body of the parent, and the egg is enveloped in a delicate membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate composed of two layers of cells (fig. 166 A and B). The two layers of this plate give rise respectively to the epiblast and hypoblast, and at a certain stage the hypoblastic layer ceases to grow, while the growth of the epiblastic layer continues. As a consequence of this the sides of the plate begin to fold over towards the side of the hypoblast (fig. 166 D.) This folding results in the formation of a remarkably constituted gastrula, which has the form of a hollow two-layered cylinder with an incompletely closed slit on one side (fig. 166 E, _bl.p_). This slit has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly, and then gradually extends forwards. In front the blastopore never becomes completely closed, but remains as the permanent mouth. The embryo after these changes has a worm-like form, which becomes the more obvious as it grows in length and becomes curved (fig. 166 F). [FIG. 166. VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS. (From Bütschli.) A. Surface view of flattened embryo at an early stage in the segmentation. B. Side view of an embryo at a somewhat later stage, in optical section. C. Flattened embryo at the completion of segmentation. D. Embryo at the commencement of the gastrula stage. E. Embryo when the blastopore is reduced to a mere slit. F. Vermiform embryo after the division of the alimentary tract into oesophageal and glandular divisions. _m._ mouth; _ep._ epiblast; _hy._ hypoblast; _me._ mesoblast; _oe._ oesophagus; _bl.p._ blastopore.]