ms. mesoblastic band; m´. mesoblast; al. archenteron; pp. body cavity.
A. Horizontal and longitudinal section of an embryo which is dividing into two embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands proceeding from them.
B. Transverse section shewing the two widely separated mesoblastic bands.
C. Transverse section at a later stage shewing the mesoblastic bands which have approached the ventral line and developed a body cavity pp.
At first the two bands end in front at the sides of the mouth, but subsequently their front ends grow dorsalwards at the expense of the adjoining epiblast cells, and meet above the mouth, forming in this way a mesoblastic dorsal commissure.
The mesoblastic bands soon travel from the lateral position, which they at first occupy, towards the ventral surface. They do not however meet ventrally for some time, but form two bands, one on each side of the median ventral line ([fig. 141] C).
The usual accounts of the origin and growth of the bands differ somewhat from the above. By Kowalevsky (No. [342]) and Hatschek (No. 339) they are believed to increase in Lumbricus rubellus and Criodrilus entirely at the expense of the mesoblasts. Kowalevsky moreover holds that in L. rubellus the original mesoblasts spring from the hypoblast. In some forms, e.g. Lumbricus agricola, the mesoblasts are not present.
In Euaxes the origin of the mesoblast bands is somewhat interesting as illustrating the relation of the Chætopod mesoblastic bands to the mesoblast of other forms. To render intelligible the origin of the mesoblast in this form, it is necessary to say a few words about the segmentation.
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 amœboid cells in the larvæ of Echinodermata, some Mollusca and other types.