322 



EMBRYOLOGY 



ever, as will be shown later, are not directly comparable to 

 the mesodermal bands of the Chtetopoda. 



As a result of the rapid cell-proliferation, the gerni bands 

 grow forward, and the layer of micromeres, which have in 

 the nieanwhile increased considerably in numbers, advancing 

 at the same time with them, covers a greater extent. Thus 

 the entoblasts gradually become overgrown by the germ 

 bands and the descendants of the micromeres. Whereas the 

 two germ bands at first diverge, their ends subsequently 

 unite at the anterior part of the embryo (Fig. 152 A). They 

 now elongate to such an extent that they occupy approxi- 

 mately the greatest periphery of the egg (Fig. 152 B). Their 

 further change in position takes place in such a way that 



Pig. 1.'2. — Embryos of Clepswe, elucidating the development of the germ bands 

 (after Whitman). Between the germ bands (fes(r) the portion already overgrown 

 by ectoderm is dotted; the entoblasts {ent) are shaded by parallel line.s. m, region 

 of the mouth; p, pole cells of the cell-rows constituting the germ bands. 



their anterior and posterior ends appear to be fixed, but they 

 themselves move down toward the ventral side, and thus 

 approach each other, so as finally to unite in the ventral 

 middle line (Fig. 152 F). Fig. 152 C, which is a view at an 

 angle of ninety degrees with that of Fig. 152 B, shows the 

 beginning of this process, whereas in Fig. 152 I) the fusion 

 of the germ bands, which takes place from in front back- 

 wards, has progressed still farther. Fig. 152 E exhibits the 

 other hemisphere, and shows that here the germ bands are 



