616 



INVERTEBKATA 



CHAP. 



will have passed iiito a new period of cleavage before another set 

 have completed the previous cleavage. The crescents of cells, neural 

 and notochordal, derived from the posterior lip of the blastopore, increase 

 to eight cells each. The two front muscle cells on each side are in- 

 creased to four, whilst the mesenchyme cells above them also divide. 



The gastrulation now begins, as in Amphioxus, by an inwardly 

 directed cytotaxis of the endoderm cells. The beginning of this 

 is no doubt really the change in shape that, as we have just noted, 

 the cells of the vegetative hemisphere have undergone, namely, from 

 a flat to a columnar shape. This change leads to a contraction of 

 their surface area, whilst the contrary change in shape undergone 

 by the ectoderm cells increases their area. 



At this stage in the proceedings there is, in front, a semicircular 

 arc, opening backwards, of eight chordal cells; and behind a semi- 



ect 



end 



end' 



end 



FIG. 445. Sections through eggs of Cynthia partita, in order to illustrate the changes in 

 form undergone by ectodernial and endodermal cells during the process of gastrula- 

 tion. (After Coiiklin.) 



Tlic small circles in the cells represent yolk-grannies. 



A, transverse section through egg vith about forty cells, showing columnar ectoderm and quadrate 

 endoderm. B, longitudinal sagittal section through egg with sixty-four cells, showing endoderm and 

 ectoderm cells both columnar. 0, longitudinal sagittal section through egg with one hundred and ten 

 cells, showing flattened ectoderm cells and columnar endoderm cells, a-t, ectoderm cells ; iml. endo- 

 derm cells. 



circular arc, opening forwards, of twelve mesenchyme cells. These 

 two arcs taken together form a complete circle inside which are the 

 endoderm cells. Beneath the mesenchymal arc lies an arc of six 

 muscle cells, whilst outside the chordal arc is an arc of eight neural 

 cells. If, however, as Castle (1894) maintains, the first cell of the 

 mesenchymal arc on each side enters into the formation of the 

 notochord, then the circle described above will consist of ten noto- 

 chordal cells in front and of ten mesenchymal cells ] >ehind. 



At a stage a little older than that shown in Fig. 444, B, the 

 neural arc described above begins to grow back, overarching the 

 chordal arc and the endoderm cells. Soon afterwards the ectoderm 

 cells lying outside the mesenchyme ring increase in number and 

 become involved in this overarching process. It follows that whereas 

 the outline of the lip of the gastrula, viewed from the vegetative sur- 



