CILETOPODA. 



267 



suppose a mistake on the part of Stossich, or that we have in 

 Chsetopods 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 meso- 

 blastic 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, 

 ?/i'), while on their inner and anterior side a row of small cells 

 appears (ms). These rows of cells form the commencement of the 



FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS 



TRAPEZOIDES. (After Kleinenberg.) 

 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. 



