Diversity of Entodciin-fonnation 163 



extend vertically until they are converted into slieets 

 enclosing tlie yolk, and finally coalesce to form the 

 mid-gut. 



In Chironomus the formation of the entoderm, as first 

 described by Ritter (1890), is somewhat different. The 

 inward -projecting ridge, at first single and median, 

 becomes paired by the formation of lateral thicken- 

 ings, and then divided by constriction into segmentallj^ 

 arranged prominences, which are almost hemispherical, 

 and bulge into the yolk. Secondary prominences (rudi- 

 ments of the mid-gut) now form upon the hemispherical 

 surfaces. These are at first segmental, distinct from one 

 another, and paired, like the prominences from which 

 they grow out ; they consist of different kinds of cells on 

 their inner and outer faces (i. e. on the faces which are 

 turned towards and away from the middle line). The 

 inner cells are relatively large, while the outer ones 

 remain small. The secondary prominences project more 

 and more into the yolk, fuse together on either side, and 

 at length become detached as a pair of longitudinal 

 bands, each consisting of an outer and an inner layer 

 of cells (fig. 121). The inner layer, which comes next 

 to the yolk, ultimately yields the mucous wall of the 

 mid-gut, while the outer layer forms the muscular wall. 

 The two bands are at first ventral to the chief mass of 

 the yolk (fig. 121), but they soon extend until they meet 

 and fuse above and below, thus completing the wall of 

 the mid-gut, and enclosing the yolk. 



It is not a little perplexing to the student that the Diversity 

 entoderm should arise in a variety of ways in different ""^ ^^*°- 

 animals. The variety of formation is illustrated by the mation. 

 fact familiar to every embryologist, that the j^olk some- 

 times lies inside the entoderm and sometimes outside it. 

 For instance, in the two primary divisions of Myriopods 

 this difference seems to be regular and characteristic. In 

 Chilopoda the mesenteron encloses the yolk ; in Chilo- 



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