By a somewhat abnormal process of segmentation the ovum divides into four spheres, of which one is larger than the others, and occupies a position corresponding with the future hind end of the embryo. The three smaller spheres give rise _on their dorsal side_ by a kind of budding to small cells, which become the epiblast; and the epiblast is also partly formed from the hinder large cell in that this cell produces by budding a small cell, which again divides into two. The anterior of the two cells so formed divides still further and becomes incorporated in the epiblast; the posterior only divides into two _which form the two mesoblasts_. The remainder of the mesoblast is formed by further division of the three smaller of the primitive large spheres, and at first forms a continuous layer between the dorsal cap of epiblast and the four largest cells which, after giving rise to the epiblast and mesoblast, constitute the hypoblast. As the epiblast spreads over the hypoblast the mesoblastic sheet gives way in the middle, and the mesoblast remains as a ridge of cells at the edge of the epiblastic cup. It forms in fact a thickening of the lips of the blastopore. Behind the thickening is completed by the two mesoblasts. The appearance of the mesoblast in section is shewn in fig. 140. As the epiblast accompanied by the mesoblast grows round the hypoblast, the blastopore assumes an oval form, and the mesoblast appears as two bands forming the sides of the oval. The epiblast travels over the hypoblast more rapidly than the mesoblast, so that when the blastopore becomes closed ventrally the mesoblastic bands are still some little way apart on the ventral side. In Euaxes the mesoblast originates in a manner which is very similar to that in some of the Gasteropoda, _e.g._ _Nassa, vide_ p. 234, and Vermes, _e.g._ _Bonellia_, etc. As mentioned in the chapter on the Mollusca the origin of the mesoblast in Planorbis, p. 227, is very similar to that in Lumbricus. Hatschek has shewn that in Polygordius the mesoblast arises in fundamentally the same way as in the Oligochæta. Besides the mesoblast which arises from the mesoblastic bands, there is evidence of the existence of further mesoblast in the larvæ of many Polychæta in the form of muscular fibres which traverse the space between the body wall and the wall of the enteric cavity prior to the formation of the permanent body cavity. These fibres have already been described in the embryo of Serpula, and are probably represented by stellate cells in the cephalic region (præ-oral lobe) of the Oligochæta. These cells are probably of the same nature as the amoeboid cells in the larvæ of Echinodermata, some Mollusca and other types. _The Larval form._ True larval forms are not found in the Oligochæta where the development is abbreviated. They occur however in the majority of the marine Polychæta.