268 FORMATION OF THE LAYERS. 



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. 



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 bauds soon travel from the lateral position, which 

 they at first occupy, towards the ventral surface. They do not how- 

 ever 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) aud Hatschek 

 (No. 339) they are believed to increase in Lunibricus rubellus and Criodrilus 

 eutii'ely at the expense of the mesoblasts. Kowalevsky moreover holds 

 that in L. rubellus the original mesoblasts spring from the hypoblast. 

 In some forms, eg. Lumbricus agricola, the mesoblasts are not pi-esent. 



In Euaxes the origin of the mesoblast bands is somewhat interesting 

 as illustrating the relation of the Chsetopod 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 foi'med 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 aud the four largest cells which, 

 after giving rise to the epiblast and mesoblast, constitute the bypoblast. 

 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 



