324 FORMATION OF THE LAYERS. 



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 some- 

 what 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 Chaetopod 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 blasto- 

 pore. 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 



