THE EARTHWORM 39 



at the anterior end, which persists and forms the mouth of the 

 adult animal. 



During these changes important developments have taken 

 place in the mesoblastic bands. At first each band consisted 

 of a single row of cells which had been successively budded 

 off from the mesomeres. As growth proceeds and the embryo 

 elongates, not only are fresh cells added to each existing row 

 by continued unequal division of the pole-cells, but the more 

 anterior cells of each row begin to divide horizontally, so that 

 each row becomes two or three cells deep in its anterior part. 

 During the formation of the enteron the anterior ends also 

 grow upwards and forwards till they meet and unite in front of 

 the blastopore, so that the last named is completely surrounded 

 by the mesoblast as it were with a ring. As the blastopore 

 narrows and closes up from behind forwards, the mesoblast 

 bands are approximated to one another, and form two nearly 

 parallel bands, one on either side of the middle line. At the 

 same time the epiblast overlying the mesoblast becomes 

 thickened. The cells of this thickened area are only one 

 layer deep, but they are columnar, and pass rather suddenly 

 into the flattened epithelium, which elsewhere forms the outer 

 layer of the embryo. As development proceeds these lateral 

 bands of thickened epiblast extend forwards till they meet 

 above and in front of the blastopore, and somewhat later they 

 send out ventral extensions, which meet behind the blastopore 

 and enclose it, forming thickened lips. These lips afterwards 

 grow into the blastopore, and form an epiblastic lining to the 

 entrance to the enteron. An epiblastic ingrowth of this kind 

 into the mouth is called a stomodseum. On the formation of 

 the stomodaeum the embryo begins to engulf the albumen 

 contained in the cocoon, and so obtains the food necessary for 

 its further development. 



The changes which follow affect the mesoblast bands and 

 the thickened epiblast lying immediately outside them. The 

 mesoblast cells in the anterior and middle parts of the bands 

 continue to divide by both horizontal and vertical divisions, so 

 that the bands become several cells deep, and more than one 

 cell thick. At the same time, each band becomes divided from 

 before backwards into a number of blocks, the mesoblastic 

 somites. The successive somites of the right and left bands 

 form pairs. They are solid at first, but shortly a cavity' appears 



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