Fig. 140. Transverse section through the ovum of Euaxes during an early stage of development. (After Kowalevsky.)
ep. epiblast; ms. mesoblastic band; hy. hypoblast.
In all the oligochætous types, with the exception of Euaxes, where the blastopore closes completely, the blastopore becomes, or coincides with the mouth. In Serpula it is stated (Stossich), as we have seen, to coincide with the anus: a statement which receives confirmation from the similar statements of Willemoes-Suhm (No. [358]). It is necessary either to suppose a mistake on the part of Stossich, or that we have in Chætopods a case like that of Gasteropods in which a slit-like blastopore originally extending along the ventral surface may in some forms become reduced to a pore at the oral, or in other forms at the anal extremity.
So far only two germinal layers—the epiblast and the hypoblast—have been spoken of. Before the invagination of the hypoblast is completed the mesoblast makes its appearance in the form of two bands or streaks, extending longitudinally for the whole length of the embryo. These are usually spoken of as germinal streaks, but to avoid the ambiguity of this term they will be spoken of as mesoblastic bands.
Their origin and growth has been most fully studied by Kleinenberg (No. [341]) in Lum. trapezoides. They commence in this species shortly before the gastrula stage as two large cells on the surface of the blastoderm, which may be called mesoblasts. These cells lie one on each side of the median line at the hind end of the embryo. They soon travel inwards and become covered by the epiblast ([fig. 141] A, m´), while on their inner and anterior side a row of small cells appears (ms). These rows of cells form the commencement of the mesoblastic bands, and in the succeeding stages they extend one on each side of the body ([fig. 141] B, ms) till they reach the sides of the mouth. Their forward growth takes place mainly at the expense of the superjacent epiblast cells, but the two mesoblasts at their hinder extremities probably assist in their growth. Each mesoblastic band is at first composed of only a single row of cells, but soon becomes thicker, first of all in front, and becomes composed of two, three or more rows of cells abreast. From the above it is clear that the mesoblastic bands have, in L. trapezoides at any rate, in a large measure an epiblastic origin.
Fig. 141. Three sections illustrating the development of Lumbricus trapezoides. (After Kleinenberg.)